That's the problem when you have a crest such as Flugplatz or the one on the Mulsanne. The aerodynamics that have been pinning the car to the floor can suddenly act as a large lifting airscoop due to the change of attack angle.
If the speed is low enough, the weight overcomes the lift generated. If you're at high speed (Flugplatz is 140+, Mulsanne Vmax) that's no longer the case, and around she goes.