While I'm sure light exposure accelerates the process, it's just an unfortunate decomposition process of some ABS formulations. IIRC it even affects products that are new old stock in the box.
I'm a bit of a vintage computer nerd, and one huge problem for collectors is Macs from around 1990 through the mid-late 90s: the plastic is becoming, as you so aptly put it, as brittle as potato chips. So people buy a lovely all-in-one from 1994 on eBay, then the seller ships it to them, and what arrives is a box of plastic bits with a motherboard and CRT sitting in it. Even if packaged correctly (which, frankly, is almost never the case...), the weight of the CRT alone is enough to crack the plastic under normal handling, never mind the torture caused by UPS.
What's interesting -- and what made me click the thread -- is that 1992 is right when something changed in the ABS formulation that causes it to embrittle far more than earlier formulations. Macs from earlier than that, like an original 1984 Mac or a Mac Plus (1986-1990) are holding up just fine, with only slight embrittlement, while the 1990s Macs are now extraordinarily fragile. Some speculate that it's due to changes in flame retardant additives, but I don't think anyone outside of the plastics industry really knows.
(All plastic Macs from the iMac in 1998 onward are made of polycarbonate, not ABS, and so will presumably have very different aging effects than their ABS predecessors.)