This is because it is grounded below sea level, and marine ice sheets such as these are susceptible to rapid melting at their base. (antarcticglaciers.org)
Thus two completely independent studies, using completely different methods, both find that the early stages of marine ice sheet instability may be underway in West Antarctica. (antarcticglaciers.org)
Joughin I, Smith BE, Medley B. Marine Ice Sheet Collapse Potentially Underway for the Thwaites Glacier Basin, West Antarctica. (antarcticglaciers.org)