I have no idea why, but it sometimes happens to me as well. For instance everything suddenly starts to behave as if a Shift key was pressed, although there was nothing pressing on either of my keyboards, and all four Shifts have clearly been not pressed.
However, I find that just pressing a Shift puts everything in order. My theory would be that this happens when the "key released" event is somewhere erroneously lost or thrown away, Maybe something similar happened here with the Page Up key? (That would be probably recognizable in other applications.)