It's called a snap.

It happens because one wing stalls before the other one, so loses its lift first. It happens when the plane slows down at the top of a loop, or, even more disturbing, when slowing down to land!

Normally only speeding up the plane, or using the rudder helps to correct the snap. Using opposite aileron also normally makes it worse as the dropped wing has to be given down aileron, which increases the drag (as lift is inversely proportional to drag), thus making the snap in that direction even worse!

It can also happen at high speeds, for slightly different reasons, but still creating different lift from each wing.

Different planes do it in varying amounts, and there seems to be a correlation between how much the LE is swept back. More sweep = more tendency to snap. I could be wrong there, but it's certainly what I've observed.

The most common cause of a slow speed snap is too much elevator movement.

-David C.