Nudge - objects move in opposite direction

[PiP]

Hello,

When I use the nudge feature occasionally the selected objects will move in the opposite direction e.g. pressing down arrow, will start moving the objects in that direction (down) but then will jump to top of the screen. 

Seems like a bug but I can't find anyone else (via google) complaining about it which make me think it's a feature that I don't understand?

Any insights would be greatly appreciated