Michael Schodlok, a JPL scientist who models the way ice shelves and the ocean interact, says melting of the underside of the shelf is a pre-requisite to these collapses. Thinning of the ice shelf reduces its buttressing effect on the glacier behind it, allowing glacier flow to speed up. The thinner shelf is also more likely to crack. In the summer, meltwater ponds on the surface can drain into the cracks. Since liquid water is denser than solid ice, enough meltwater on the surface can open the cracks up deeper down into the ice, leading to disintegration of the shelf. The oceans surrounding Antarctica have been warming 10, so Schodlok doesn't doubt that the ice shelves are being undermined by warmer water being brought up from the depths. But he admits that it hasn't been proven rigorously, because satellites cant measure underneath the ice. 
West Antarctica is a series of islands covered by ice. Think of it as a frozen Hawaii, with penguins. (Image credit: National Science Foundation)
http://www.nasa.gov/topics/earth/features/20100108_Is_Antarctica_Melting.html