Seems to be the Leading edge Slats being confused when stationary/low speed on the ground - the jet registers that it's slow and wants to stall (and the slats angle down in anticipation) but it instantly corrected by the ground propping it up - slats flick up.

This cycle happens repeatedly and rapidly until airspeed is built up, jet is shut down, etc.

Suggest adding a momentary timer to detect direction of 'travel' of the jet over a short period of time, or making the animation happen slowly enough the constant up/down corrections aren't noticed)