it is equipment failure related..... bad bearing in a motor, blade balance out of whack, broken pcb inside the naza unit from crashing. The problem is people start with naza so it takes a beating and theres millions of them.
If you do everything perfect it'll behave, but man, that's a long list. takes some getting used to the craft in general and how to maintain them.... I've ran naza since they came out, lots of em