Why do tanks crash?

[christyf5]

two partial crashes, one a SE bulb broke the outer casing and continued to work, irradiating one side of the tank with UV killing 3 fish and a multitude of corals.

Second crash was due to reefer stupidity (not listed, I selected temperature deviation): moving the tank at the end of the day (an all day move) and left the livestock on a cool concrete floor (it didn't feel all that cold at the time) while I took a dinner break. Then put the corals into the tank without matching the temperature. The colonies in the coolest water started to RTN and caused a cascade effect that took 2 weeks of massive waterchanges to get the ammonia down to an acceptable level where it wouldn't cause issues for the rest of the corals. I lost most of my larger colonies, didn't lose any fish though (they were in an insulated cooler).