Beard Creep

I suffer from something I call beard creep, and I suspect all men who trim their beards under their chins also suffer from the same condition.

Beard creep is when you trim the beard hair under your chin, which is close shaven anyway (as in stubble length), along the neck and, slowly but surely, the position if the trim point creeps down along the neck such that two or three weeks later, you wonder why the hell your beard is halfway down your throat, whereas you initially trimmed it be be just under your chin.

It's not the hair from the original trim length growing, as that's kept deliberately short with a zuzzer. It's the actual trim line that creeps.

I can't figure out why it happens.