In all seriousness, can a fly unzip by itself?
I just noticed mine was down and this came to mind.
First off, I love your username!
And yes, I have often noticed this phenomenon in ill-fitting (too tight) pants, although I had one pair of well-fitting-to-overly-large pants that would also magically unzip themselves. So it could be just an aberration with that particular pair, but it might also be useful to just generally make sure that you have a proper fit in your clothes.
Hope this helps!
Pink Floyd’s Wish You Were Here is one of my favorite albums. 
As a matter of fact, the jeans I am wearing are a bit on the tight side.
Doing the mouse-over, I thought, “The little winged btds don’t wear pants.”
Whoopsie.
Regarding the OP, yep. They certainly can unzip by themselves. Constantly moving around, walking, sitting, and standing caused this to happen to me many times. Stupid pants.
Come on, you boychild, you winner and loser…
In case you weren’t aware, with certain flies (especially those on jeans), you have to have the head (?*) of the zipper pointing due south, which locks it in place and prevents it sliding down on its own.
*I have no idea what that thing is called: the part of the zip that you slide up and down the teeth to zip and unzip them. If it has a name I’m blanking on it.
From here: http://www.zippersource.com/parts/definitions.asp
PULL TAB: The part of the slider that you hold to move the slider up or down.
SLIDER: The device that moves up and down the chain to open or close the zipper.