That makes sense regarding the preservation. The specimens who just die on the ground are bound to be torn up by scavengers. But being caught in a flash flood could kill and quickly bury it.
The fossil record of the trilobite (Wisconsin’s state fossil) makes very interesting reading. Uncountable numbers of them are preserved, whole and undamaged, in some sites their chitinous exoskeletons preserved (not mineralized), obviously mass burials as by a major flood.
/bingo