Both tectonic and seismic data suggest that horizontally wide (in the direction parallel to the axis of the trench) dipping subducting slabs typically move stably at fixed angles to the horizontal, and that their associated trenches migrate very slowly, or not at all. In contrast, where backarc basins (BABs) exist, the associated trenches typically migrate at rates of several cm/year. This behavior may be related to the finite width of the slabs under BABs. Analysis shows that hydrodynamic suction created by the downdip motion of a wide slab is sufficient to balance the pull of gravity on the heavy slab. However, for a narrow slab, sideways asthenospheric flow into the slab/overriding plate corner effectively eliminates the suction, which results in the rapid sinking of the slab. Hydrodynamic modeling is used to quantify this hypothesis. Modeling is used also to show that wide slabs are stable in their inclined position not because of the gravitational wing-like sliding but due to the lifting force produced by the negative pressure (suction) in the slab/overriding plate corner. Finally the model results confirm that for relatively narrow slabs, this lifting force is small and cannot counterbalance gravity. Therefore, when bending moments in a narrow slab exceed the plastic yield limit, the slab becomes unstable. This instability can cause the rapid migration of the slab's hinge and the associated arc at rates of 10-20 cm/year and thus form the BAB.