The force is driven by the negative buoyancy of the slab. If the slab detaches, there is no more driving force for the portion of the plate on the surface whereas the slab continues to sink. A simple analogy would be a weight clipped to the edge of a floating mat. If the mat rips, the portion attached to the weight will sink but the rest of the mat will just sit there (assuming it is buoyant). This is expanded on in much more detail in a top level answer I made within this thread to try to address the relative incompleteness of the specific top level comment that everyone is upvoting.