When to explain poor performance in a Statement of Purpose I know there have been similar questions asked, but the most recent question was focused on the transcript and the less recent question was focused on courses not relevant to the major. 

My general question:
When is it a valuable addition to a Statement of Purpose to explain an isolated case of poor performance in important coursework? 

My specific case:
I am an undergraduate currently writing a statement of purpose. I have had a 4.0 in math up until last semester. Last semester I took the following courses (with corresponding grades):

Graduate Real Analysis 1 (B) 

Graduate Differential Topology 1 (A) 

Graduate Probability Theory (A) 

Undergrad Combinatorics (A) 

Undergrad gen ed (A) 

My explanation would include things like:


I was working 15 hours a week
I spent multiple hours a week applying for a Grant (and doing some preliminary Research along the way) and applying for the Goldwater scholarship
I was naive and overconfident to think I could go beyond a normal graduate course load (or something to that effect) 


My reason to think that an explanation may be worthwhile in my case is because the B was in a very important graduate math course and I feel that, in effect, I just displayed that I hit my limit and my limit is below graduate level rigor. In reality, with my explanation, I feel like I may be able to convince someone that I actually was beyond the rigor of a first semester in graduate school and was simply naive and over confident.
My general question: When is it a valuable addition to a Statement of Purpose to explain an isolated case of poor performance in important coursework? 


Among scenarios I can imagine, never.  You don't want to place extra attention on potential mild weaknesses in your application.

As mentioned in the comments, in your particular case one B in a grad class while you were taking 2 other grad classes and 2 other math classes with all As doesn't really look bad.  The normal interpretation would be this student didn't have enough time to get an A in that one class with all the other work.

For the general question, one situation you might want addressed is if you have a semester that really looks bad (light load and actually bad grades, e.g., F, D, C), maybe due to major illness or personal tragedy.  However, the statement of purpose doesn't really seem the right place to address it.  A good letter writer will know when and how to explain this in your references, so you shouldn't need to explain in yourself in your application (though you may need to explain it to your letter writers if they're not aware).