I think the peg on the top plate is jamming with the slot in the twistgrip. Not Jammin, Jammin, but Jamming

Did you move or loosen the switch unit on the bar at all? My bike has been down on its RHS at some point and the end of the handlebar was a bit chavelled (inside the twist grip). if you slid the switch unit and twist grip too far on, it would catch on the end of the bar - the tightening of the top plate making it stick.