Some attentive soul could have known in advance that the limousines wheelbase was too long, and the clearance too low, and that the vehicle would have become stuck on that speedbump. That soul could've taken advantage of the fact that the President of the United States was stuck in an exposed, vulnerable, stationary position outside the routine contingencies, and could've used that advantage to cause chaos.
Thank God nothing happened to the president during that prolonged moment of vulnerability.