user8157: In researching Dependent Origination as described in SN 12.2 and using a basic Pali language guide (page 18), I found the following about each main Pali word:

jarāmaraṇaṃ singular from maraṇa neuter
jati singular from jāti feminine
bhavo singular from bhava masculine 
upādānaṃ singular from upādāna neuter
taṇhā singular from taṇhā feminine
vedanā singular from vedanā feminine
phasso singular from phassa masculine
saḷāyatanaṃ singular from saḷāyatana neuter
nāmarūpaṃ singular from nāmarūpa neuter
viññāṇaṃ singular from viññāṇa neuter
saṅkhārā plural from saṅkhāra masculine
avijjā singular from avijjā feminine

The above seems to correlate with every translation I have read. Note: saḷāyatanaṃ may appear to be translated as plural but it appears singular because it includes 'six'. 
For example, all translations say: "What is feeling [singular]?" rather than "What are feelings [plural]?" 

And what is feeling? Katamā ca, bhikkhave, vedanā? There are these six
  classes of feeling. Chayime, bhikkhave, vedanākāyā— Feeling born of
  contact through the eye, ear, nose, tongue, body, and mind.
  cakkhusamphassajā vedanā, sotasamphassajā vedanā, ghānasamphassajā
  vedanā, jivhāsamphassajā vedanā, kāyasamphassajā vedanā,
  manosamphassajā vedanā. This is called feeling. Ayaṃ vuccati,
  bhikkhave, vedanā.

The saṅkhārā condition is described as follows:

And what are saṅkhārā [plural]? Katame ca, bhikkhave, saṅkhārā?
There are three kinds of saṅkhārā. Tayome, bhikkhave,
  saṅkhārā— body saṅkhāro [singular], speech saṅkhāro [singular] and mind saṅkhāro
  [singular] kāyasaṅkhāro, vacīsaṅkhāro, cittasaṅkhāro.

Why is the Pali word 'saṅkhārā' in Dependent Origination plural where all other words are singular? 

user10100: Dependent origination is the cycle of the happening event. Each event has only one decision. There are 2 saṅkhāra for each decision: wholesome and unwholesome. Wholesome saṅkhāra gives good resultants. Unwholesome saṅkhāra gives bad resultants. So, viññāna to vedanā which are the resultants of saṅkhāra must be singular, because it's just the resultants of one decision. Jāti and jarāmaraṇaṃ are same. Wholesome karmma-bhavo gives good resultants. Unwholesome karmma-bhavo gives bad resultants. So, jāti and jarāmaraṇaṃ must be singular because of the same reason.
Above explanation explained clearly in abhidhamma vibhaṅga paccayākāravibhaṅga and it's atthakathā. It separates each event to singular as "saṅkhāro". That's why no direct explaination in any atthakathā, included path of purification. However, it's used widely in difference context such as in VN.A. Introduction.
And this is the example of what I wrote "it's used widely":

Kusalattika: kusalā dhammā, akusalā dhammā, abyākatā dhammā.
The "wholesome" triad: wholesome phenomena, unwholesome phenomena, not
  both phenomena

The Path of Purification detailedly exposited the dependent origination follow to 'the "wholesome" triad':

jarāmaraṇaṃ = not both phenomena
jati =  not both phenomena
bhavo =  not both phenomena
upādānaṃ = unwholesome phenomena
taṇhā =  unwholesome phenomena
vedanā =  not both phenomena
phasso = not both phenomena
saḷāyatanaṃ =  not both phenomena
nāmarūpaṃ = not both phenomena
viññāṇaṃ = not both phenomena
saṅkhārā = 1. wholesome phenomena, 2. unwholesome phenomena
avijjā = unwholesome phenomena

You can see whether saṅkhārā are separated into 2 events.
Above quote is my own conclusion. To read it, the reader needs to understand in advance of chapter XIV, XV, XVI, XVII, and some part of chapter I, III. Also, it requires Abhidhamma knowledge in advance, as well.
Path of Purification, CH. XVII, THE SOIL OF UNDERSTANDING—CONCLUSION, DEPENDENT ORIGINATION, III. Detailed Exposition page 547-608.

8157: <p>In researching Dependent Origination as described in <a href="https://suttacentral.net/sn12.2/pli/ms" rel="nofollow noreferrer">SN 12.2</a> and using a basic Pali language guide (<a href="https://host.pariyatti.org/plc/EngPaliGrammar_w.audio.pdf" rel="nofollow noreferrer">page 18</a>), I found the following about each main Pali word:</p>

<blockquote>
  <p><strong>jarāmaraṇaṃ</strong> singular from maraṇa neuter</p>
  
  <p><strong>jati</strong> singular from jāti feminine</p>
  
  <p><strong>bhavo</strong> singular from bhava masculine </p>
  
  <p><strong>upādānaṃ</strong> singular from upādāna neuter</p>
  
  <p><strong>taṇhā</strong> singular from taṇhā feminine</p>
  
  <p><strong>vedanā</strong> singular from vedanā feminine</p>
  
  <p><strong>phasso</strong> singular from phassa masculine</p>
  
  <p><strong>saḷāyatanaṃ</strong> singular from saḷāyatana neuter</p>
  
  <p><strong>nāmarūpaṃ</strong> singular from nāmarūpa neuter</p>
  
  <p><strong>viññāṇaṃ</strong> singular from viññāṇa neuter</p>
  
  <p><strong>saṅkhārā</strong> plural from saṅkhāra masculine</p>
  
  <p><strong>avijjā</strong> singular from avijjā feminine</p>
</blockquote>

<p>The above seems to correlate with every translation I have read. Note: saḷāyatanaṃ may appear to be translated as plural but it appears singular because it includes 'six'. </p>

<p>For example, all translations say: "<em>What is feeling [singular]?</em>" rather than "<em>What are feelings [plural]?</em>" </p>

<blockquote>
  <p>And what is feeling? Katamā ca, bhikkhave, vedanā? There are these six
  classes of feeling. Chayime, bhikkhave, vedanākāyā— Feeling born of
  contact through the eye, ear, nose, tongue, body, and mind.
  cakkhusamphassajā vedanā, sotasamphassajā vedanā, ghānasamphassajā
  vedanā, jivhāsamphassajā vedanā, kāyasamphassajā vedanā,
  manosamphassajā vedanā. This is called feeling. Ayaṃ vuccati,
  bhikkhave, vedanā.</p>
</blockquote>

<p>The saṅkhārā condition is described as follows:</p>

<blockquote>
  <p>And what are <strong>saṅkhārā [plural]</strong>? Katame ca, bhikkhave, saṅkhārā?</p>
  
  <p>There are three kinds of saṅkhārā. Tayome, bhikkhave,
  saṅkhārā— body saṅkhāro [singular], speech saṅkhāro [singular] and mind saṅkhāro
  [singular] kāyasaṅkhāro, vacīsaṅkhāro, cittasaṅkhāro.</p>
</blockquote>

<p>Why is the Pali word 'saṅkhārā' in Dependent Origination plural where all other words are singular? </p>
 10100: <p>Dependent origination is the cycle of the happening event. Each event has only one decision. There are 2 saṅkhāra for each decision: wholesome and unwholesome. Wholesome saṅkhāra gives good resultants. Unwholesome saṅkhāra gives bad resultants. So, viññāna to vedanā which are the resultants of saṅkhāra must be singular, because it's just the resultants of one decision. Jāti and jarāmaraṇaṃ are same. Wholesome karmma-bhavo gives good resultants. Unwholesome karmma-bhavo gives bad resultants. So, jāti and jarāmaraṇaṃ must be singular because of the same reason.</p>

<p>Above explanation explained clearly in abhidhamma vibhaṅga <a href="http://www.84000.org/tipitaka/pitaka3/v.php?B=35&amp;A=3732&amp;Z=3845&amp;eng=suttacentral" rel="nofollow noreferrer">paccayākāravibhaṅga</a> and it's atthakathā. It separates each event to singular as "saṅkhāro". That's why no direct explaination in any atthakathā, included path of purification. However, <strong>it's used widely</strong> in difference context such as in VN.A. <a href="http://www.84000.org/tipitaka/atthapali/read_th.php?B=1&amp;A=2430&amp;w=&amp;pagebreak=1#p116" rel="nofollow noreferrer">Introduction</a>.</p>

<p>And this is the example of what I wrote "it's used widely":</p>

<blockquote>
  <p><a href="http://www.84000.org/tipitaka/pitaka3/v.php?B=34&amp;A=1&amp;Z=103&amp;eng=suttacentral" rel="nofollow noreferrer">Kusalattika</a>: kusalā dhammā, akusalā dhammā, abyākatā dhammā.</p>
  
  <p>The "wholesome" triad: wholesome phenomena, unwholesome phenomena, not
  both phenomena</p>
</blockquote>

<p>The Path of Purification detailedly exposited the dependent origination follow to 'the "wholesome" triad':</p>

<blockquote>
  <p>jarāmaraṇaṃ = not both phenomena</p>
  
  <p>jati =  not both phenomena</p>
  
  <p>bhavo =  not both phenomena</p>
  
  <p>upādānaṃ = unwholesome phenomena</p>
  
  <p>taṇhā =  unwholesome phenomena</p>
  
  <p>vedanā =  not both phenomena</p>
  
  <p>phasso = not both phenomena</p>
  
  <p>saḷāyatanaṃ =  not both phenomena</p>
  
  <p>nāmarūpaṃ = not both phenomena</p>
  
  <p>viññāṇaṃ = not both phenomena</p>
  
  <p>saṅkhārā = <strong>1. wholesome phenomena, 2. unwholesome phenomena</strong></p>
  
  <p>avijjā = unwholesome phenomena</p>
</blockquote>

<p>You can see whether saṅkhārā are separated into 2 events.</p>

<p>Above quote is my own conclusion. To read it, the reader needs to understand in advance of chapter XIV, XV, XVI, XVII, and some part of chapter I, III. Also, it requires Abhidhamma knowledge in advance, as well.</p>

<p><a href="https://www.accesstoinsight.org/lib/authors/nanamoli/PathofPurification2011.pdf" rel="nofollow noreferrer">Path of Purification</a>, CH. XVII, THE SOIL OF UNDERSTANDING—CONCLUSION, DEPENDENT ORIGINATION, III. Detailed Exposition page 547-608.</p>
