Multi-rotor crashed suddenly while in air

Future crashes are avoidable if the user reads the How to methodically configure and tune any ArduCopter blog post, especially section 1.1 of it.
All of these issues are addressed there.

Users should be asking questions about specific parameters, and not asking why their vehicle crashed.

Crashed vehicles is very avoidable if you follow the publicly available, simple to follow documentation.

AFAIK the hard part is understanding the parameters and their effect.