I would attest this to a bad connection except that it happens every match I've been in so far, and my connection's pretty good!
A minion, at some point in the game (sometimes it's scuttler this happens to), will stop moving. the rest of the wave will move, but a lone pacifist minion will stop all actions and refuse to participate in battle. It still takes damage, but it won't move. It ends up dying. In the event it isn't killed, it will remain stationary until killed.
I don't have a way to replicate this -- it's been happening in all of my games though. It doesn't happen every wave either.