To enlarge a bit upon Carl Witthoft's answer, which I upvoted, remember that the swivel mechanism in the skateboard's wheel trucks is designed to provide a certain (fixed) amount of steering action for a given amount of board tilt. At ordinary board speeds (a brisk run) the board tilt provides the rider with approximately the right amount of lean angle to make the turn coordinated (as it would be for instance on a bicycle in a turn).
The rider has no way to decouple his lean angle from the board tilt angle to produce a coordinated turn at speeds different from the board's "design speed"- meaning that at slower speeds, the rider will tend to fall towards the inside of the turn and at greater speeds he will tend to fall towards the outside of the turn.
At high speeds, fighting this tendency puts the board into a speed wobble in which the upper part of the rider's body does not track the movements of the board while the lower part does. Mayhem ensues.