reason for degradation - loss of aromatics from the plastic (yes an elastomer in a bike was plastic, no natural rubber here folks, dinosaur extract all the way, drill baby drill) results in chemical bond structure and lattice changing, elastomer becomes hard.
stick it in the microwave 2 things happen, one those bonds open up with heat and everything becomes more pliable again (more room for the sneaky particles to get past each other), two some of those aromatics that were left behind run away too (so your microwave now smells like a dinosaur fart). wanna guess what happens when you remove more of the flex giving, loose bonded aromatics from the long chain that is a plastic? yep you guessed it, less flex. depending where they are in the chain, you end up with brittle (because there's a hole now you see) plastic eventually.
TLDR - dinosaur farts smell like plastic.