Damn fast, panic sunning basically.
I can explain this and I'm surprised you never noticed this when playing on a melee character.
If you hold down the right click button behind someone and at the same time run INTO them, you can get hit once even twice, flinch, but if you then move your mouse off in a different direction quickly after you'll start running. It happens to mobs too. Happens to all classes but is mostly recognised by melee's. Really hard to reproduce, happens a lot more often with mobs than it does players.
Xele actually made a report of this early on in the server, but I don't believe it was ever fixed or looked at.
Here you go, video of it in action.
[video=youtube;_mOg-E2Hj44]https://www.youtube.com/watch?v=_mOg-E2Hj44&feature=youtu.be[/video]