I've noticed this too. Sometimes one of my characters would just start running like his auto run was turned on.

At first I thought it might have been because there's delay between the target change on the main and the focus target that appears on a slave. This might cause a slave to target something else and go for it. However I have yet to pit point a reason for the problem.

We could try posting it in some bug report on the wow forums, might get some attention. Although it's not game breaking, and it doesn't happen all the time, so I doubt any developer will bother. Solo players don't really bother with follow anyway.