My tank crashed due to a design flaw, which led to an electrical failure, which led to equipment failure, which led to a temperature deviation, which led to low oxygen levels, which led to a water chemistry change, which led to disease and parasites, LOL. Seriously though, in my experience it tends to be a cascading series of unfortunate events which does stem from one or more points of origin.
The sad part is, sometimes (or most of he time), it can be hard to pin point exactly where it all started to go down hill..
edit: wasn't referring to my current tanks BTW. Rather, previous tanks I've had. I've tried to learn from those past mistakes, and so far so good *knock on wood*