"The cause", singular, shows you're not getting the the swiss cheese model still. Trying to find a singular cause is a big mistake which prevents other causes from being addressed.

Every hole in the cheese slices, including operator error, is important. And operator error is an important kind of error in particular, not because we need to prosecute somebody but because many forms of operator error can be mitigated using better system design, but you won't figure that out if you blind yourself to operator errors.