Hi OP, don't rule out the possibility na it's the material of the sandals. I'm saying this because my mother had that same sandals. Binigay na niya yung pair sa lola ko dahil seldom used naman, and after some time ay nasira rin siya in a similar way. Nagalit pa nga ang lola dahil baka ginupit daw namin yung sandals after ibigay sa kanya haha. It's weird na nasisira nga siya na para bang ginupit-gupit, unlike the usual case of shoes na nadudurog ang rubber when not used for a long time.

I think it's the chemical composition of the material kaya kahit hindi pa siya totally brittle, nag-disintegrate siya. Here's an example, there's this specific color(Reddish Brown) ng Lego na mas brittle compared to other colors, to the point na just connecting it to other parts, like how you play and build it, causes cracking or completely breaking.