As it happens today I've binned an 'Addis Cutlery Drainer' that's had a hard year of occasional sunshine UV exposure holding our toothbrushes.
It's crumbling similarly and bits are now easily snapped off. I'd say at a guess it's made of the same type of disintegrating cheese where I think (looking at google) they may have left out the necessary UV stabilising additives. These things are meant to have a 10 year guarantee!