i've been having this problem as well, and it seems it's always when i use my formation (which i often use just to break /follow)

Basically with my 3 toons, the formation key is '\' (backslash)
This is bound to DOWN on my main, STRAFELEFT on clone1 and STRAFERIGHT on clone2.

Seems sometimes clone2 decides to run forward instead. looks like it receives AUTORUN instead of STRAFERIGHT basically.

From memory it's always the same clone that runs like a fool.
Pressing the follow key (backspace in my case) usually bring him back if i'm quick enough.

i had the one time when he would turn and run back, but continue to run past my other toons until i pressed follow again, then turn again and run past us again etc.