Q:

Why does the D string in a guitar breaks so often?

Well... that's about it. D strings (on acoustic nylon guitars at least) break way more often than the others.
I guess there's a physical explanation but I wasn't able to found anything on the Internet. Do you guys know why this happens?

A:

First, I agree with the question, when talking about nylon-stringed guitars - in nearly a half-century of playing classical and flamenco instruments, I find that the D string, the poor thing, breaking more frequently than any the others (other answers and comments are probably based on steel-stringed experience). I've asked luthiers, and even one of the D'Addario sons   :-) about this, and there are a number of causes (besides flaws in the guitar itself):

The D is a wrapped string, and more likely to break than a solid string
It has thinner wire than the other two bass strings
It is under the highest tension of the three bass strings
It gets more wear than the other three (think about melodies you've played on that string - it's the brass section of the guitar).

Of course, replacing your basses frequently will usually save you the embarrassment of changing strings on stage (I had to do it twice with a D string recently, because I botched the first stringing job - oh well). The goal is to keep them from breaking, until they finally "wear out" - the basses start to sound like rubber bands. So what can you do about that?

As I said, change the basses before one breaks. Besides the sound going bad, look for the finish wearing off. And here's a trick - if you take them off before they get really worn, you can put them back on again later, and they will not only sound great for a short while, but because they are "pre-stretched", they will settle in tune much more quickly. It's very helpful to have a "rested" set of basses like this to put on just before a gig (or, gulp, during one...).
Make sure that the loop at the bridge is as close as possible to the wood - look at the bass strings in the picture below, note how snug against the wood the loop is. Push it back and down with your nail as you tighten the new string. If you leave it floating half-way between the wood and the ivory, it is putting a lot of pressure on the middle of that short stretch of string, essentially trying to cut it in half. My D's would often break right there, until the luthier/engineer John Gilbert told me what's what.

If the D string still tends to break at that spot before it wears out - perhaps because of the construction of the bridge, or because you are using very high tension basses - you can do what I do on a particular instrument, and protect the D string with a very short piece of narrow rubber tubing that can cover the D between the wood and the ivory - the loop cuts into the rubber, but not the string. And where does one find such a thing? Why, strip the insulation off some wiring that's slightly thicker than your D!

Good luck, and may your bass strings always maintain their composure when you are playing for people...

A:

IMO frequently broken strings indicate a mechanical problem. I never break strings and I haven'tt broken one for maybe 30 years.
Causes include: 

Too-sharp edge on nut or saddle.  
burr or sharp edge on a tuning post, or the hole though same.
Nut slots cut too wide (or maybe you installed lighter strings) allowing the string too much side-to-side movement.

etc, etc...
Of course, if you really flail away at the strings while strumming... 

A:

From m practice, This is definitely true for nylon strings and this happens because of structure of D-string. The first 3 (E,B,G) are solid, and next 3 (D,A,E) consist of thin nylon fibers wrapped around one more think string. For the D-string the number of fibers is much less then for A and E (because it needs to vibrate in certain range) and this makes it the weakest.
When I would buy strings, I'd always buy couple of spare D-strings...

