It can be caused anytime the rear wheel slides and then suddenly regains traction. Either too liberal an application of throttle or too much rear brake and then suddenly letting off of either.

Many times a highside can be avoided by simply maintaining your throttle position (or backing off as little as a couple %) as the bike begins to slide and letting it naturally scrub off speed. What happens is (especially newer) riders let off the throttle too suddenly, the rear tire immediately regains traction which in turns bucks the rider off of the bike.

Since the inclusion of traction control in MotoGP highside crashes have nearly been eliminated.