The intentions are usually the best. A beard is slightly overgrown, a quick touch up will make it look better. That's the motivation. But other considerations enter into the equation. A steamed up mirror. An over zealous gesture of the wrist. A dull razor blade.
But undoubtedly the most precarious factor is the human eye. A hypothetical situation:
A man enters into the bathroom intending on trimming his full beard. The hair has grown too high onto his cheeks, and it looks unkempt and half-cocked. The tool of choice: a three-bladed Gillette razor. The initial debate: does he start high and work low, or start low and go up?
The shaving cream is applied, and whether he starts high or low, the same thing seems to happen: after shaving and washing the cream away, one side is a little askew compared to the other. More shaving cream is needed... and then the follicular formation is lopsided in the other direction. Repeat. Same problem.
Before the victim knows it, their beard is all but gone. Very little reason to keep the goofy-looking stray loners poking out all over the face. And so, the fateful, heavy decision is made: the facial hair must go altogether. The departure takes place with the hope that a new dawn will see the beginnings of stubble. With each subsequent dawn, hope will swell. The newly shorn man comforts himself with that knowledge, that God has worked into his DNA the automatic capability for hairy growth.
Until the beard returns... don't overdo it.