Well I'm not a chemist but...these are manufactured from who knows what random materials...in giant quantities...where quality is something not even mentioned. So the outcomes probably are random.
I just came across a melted elastic last week! Never saw it before either and I was totally grossed out by it, but tried to reason it away with "might have put it into the sun/on the heating" but really didn't believe myself. Now you encounter the same thing which probably means that we both don't imagine elastics suddenly melting. Gross.
I've seen this in both elastics and clothes made from elastic fabric. It seems to me that the plastic/rubber/whatever (chemistry really isn't my strong suit) that makes it elastic deteriorates over time.
I second Anonymous' motion. It's just that rubber and plastic unfortunately age terribly, thus the melting or crinkling. It's produced in bulk and hardly ever of good quality.Perhaps silicone ones age better? ;-)
I've had a few go gross on me, but I can't be sure something didn't spill on them to cause it ;)
I wonder if some of it could also be that you put oil on your hair? Oil can degrade rubber, and i think silicone too? Not sure on that last one. I've noticed that any rubber that comes in contact with essential oil in particular will just melt into an indistinct goo.