I read that this is an issue about polyurethane, the plastic material from which most modern boots and shoes are made. Polyurethane is porous and, within years of production, and even if the shoe was never used, air seeps through and made the soles crumble.
This issue is some sort of a nightmare for hardcore sneakers collectors, who might pay thousands for a pair of never used Air Jordans from the 1980s, only to find out that their soles are falling apart at the slightest touch.

Here's a link to article in Wired on the matter.

https://www.wired.com/2015/05/sneakers/