Very strange! Did you store the Playmobil in cardboard boxes or plastic containers?
Cheap toys made of cheap plastic sometimes pulverize after a few years, due to the composition of the products. But good quality toys such as Playmobil and Lego can really last for decades, meaning your grandchildren will still enjoy them.
What I always understood is that certain oils are used in the plastics of Playmobil and Lego that contribute to this. Could it be that those oils are leaking from your Playmobil? That would be very special! I'm no physicist, but that seems very strong to me, especially if it is stored well packaged in your attic.
Normally it is better to store your Playmobil in the attic than in the basement, due to moisture. It can of course get warm in an attic, but so hot that a plastic melts seems impossible to me.
So it would have to be something from outside, but could moisture, for example, be added? Or do you smoke a lot?
But normally Playmobil, just like Lego, lasts an awfully long time (except for the Wiltopia, the organic Farm and the new World of Horses, which are of rubbish quality, which is so breakable and does not last long, so far the durability of "sustainable" ;-P)
...Good story, nice and short!