Antarctic Mountains

Apparently scientists are studying the ice sheet covering Antarctica. Among other things they've learned that there exists a large mountain range that rivals the Alps in height. The interesting thing to me was the following:

The mountains would probably have been ground down almost flat if the ice sheet had formed slowly. But the presence of jagged peaks might mean the ice formed quickly, burying a landscape under up to 4 km (2.5 miles) of ice.

Now, how could that have happened?