Why does strong shock wave-boundary-layer interaction produce a bubble reattachment? The picture represents a lambda type shock wave, separating the boundary layer at its foot. Separation causes a recirculation bubble to appear. I understand why separation has happened, but what is the exact reason for a bubble reattachment?

The picture is from the book called Shock Wave-Boundary-Layer Interaction by Holger Babinsky and John K. Harvey, Cambridge University.
After the rear shock leg, the subsonic flow inside the boundary layer decelerates further, which causes an expansion of the streamtubes and therefore reattachment. Note that with the presence of the supersonic tongue, the deceleration into the subsonic regime happens somewhat "slower" inside the boundary layer than outside (especially above the slip line).