It's just flow from the top, and very sudden freezes. I copied this from one of the descriptions:
"Exposed and unpredictable in 1936 it was one of the nations most significant step-cutting alpine routes. In 1969, it saw the unveiling of Yvon Chouinard's new steeply drooped ice axes. Even with todays new gear the slab is a risky outing. Protection and belay anchor quality can be fickle. Avalanches routinely drop from the upper slopes and have taken parties to the ground. Water pressure can be released from under the ice with serious consequences and happens often on the slabs. This is some of the best easy-moderate ice in the area but involves a great commitment on the short winter days."
bart