If I maintain a decent speed and lower my center of gravity slowly by bending my knees, it will start to wobble. I'm 6' tall. It also starts pretty easy if I'm tip toeing on the pedals.
Keeping some weight planted uniformly on the pedals helps to keep it from starting. Once it starts I have to do the usual tricks to stop it: adjust center of gravity, put most weight on one pedal, or carve.
I think it starts oscillating when one pedal dips, pushing the other pedal with weight on it up, which forces the other pedal back down again too fast, which forces the other pedal up again, and so on.