Its a problem that is obviously of some concern, personally I would put it down to the plastisizer (lubricant) leaching out from basically a chemically unstable mix, that old plastic smell, theres a whole bunch of reasons why this may happen sooner or later and likely its a combination of events....
However I've had many plastic household items go rotten over the years as well, much more so then my soft plastic figures and in a much shorter time, some of my collection is over 50 years old and just as flexible as ever!
"This thing all things devours:
Birds, beasts, trees, flowers;
Gnaws iron, bites steel;
Grinds hard stones to meal;
Slays king, ruins town,
And beats high mountain down." J.R.R Tolkien.