Nfk, I did a bit of Googling, and it seems that shoes made by Hotter, Ecco, and Clarks, to name but a few, Often crumble away if not worn enough!
It's the material the sole is made from PU, light and comfy, but needs constant compression to get any liquid out of them, and kind of dry them out.So, if you buy a pair, wear them straight away and keep wearing them for a while, don't stick them in a cupboard.I think that once you have worn them a lot of times, it may be safe to put them away for a while, but not even sure of that!The more expensive shoes at M&S do the same.