A lot of things can cause a tank slapper
Setting a wheelie down with the tire not straight, Tight on the bars, a bad road, bad tire
Basically, a tank slapper is the front tire “looking” for traction. (I guess traction is the word).
In Twist of the Wrist II, you can see immense tire deformation from the front tire coming down on a wheelie, and it tries to fix itself.