Catching air under the wheel and it hitting anyways but straight. Also accelerating really hard over uneven pavement. Lots of things can cause them honestly. Next time riding, shake your bars a little bit at speed and you'll feel uneasy. Now imagine that x10. They can get pretty violent. I've experienced so many tankslappers while hitting corners or accelerating over bad asphalt that it's an uneasy experience. I need to purchase a steering damper.