The important oddity is that as you move further inward, the land is further below sea level. Ice flows from the grounded part into the floating part. This makes the ice sheet just a little thinner. Without that mass, what used to be ice sheet begins to float. Since the sea floor slopes down inland of the grounding line, the area of ice sheet that turns into ice shelf increases rapidly.
More ice shelf means more chance for ice to be melted by the ocean.
The collapse mechanism has a mirror-image advance mechanism. Go back to near the grounding point. This thickens the system to ground ice shelf. The grounded ice shelf takes area away from the ocean ablation zone, which makes the mass balance even more in favor of accumulation. So the advance can also be a self- acclerating process.