because
a) they are of a type that can absorb microwaves and therefore the microwave heats them
or
b) they absorb heat from whatever is place on them in the microwave
or
c) the plate has been placed in the microwave wet and the material of the plate has absorbed water in places
In all these cases the plate heats unevenly and therefore it breaks
I use my microwave to heat plates and haven't lost one yet.Some of them are old and made before microwave machines were invented so they were hardly designed to be microwave safe.