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So if I went up to where this rock slide originated sometime last week, my guess is that I would have seen a surface of mostly solid rock. That is I wouldn't have seen the relatively small stones that are seen flowing down the mountain here. My question is: what causes the rock to fragment like this?

ML/NJ

First, the relatively small rocks you refer to ... some of those would be the size of your car. This mountain is 14,410 feet high, and the picture is probably being taken from around the 7,000 ft level.

I cant be sure exactly where they are, but my guess is that this was actually part of the Ingraham Glacier calving over the rock face that separates it from the Cowlitz Glacier. As the tons of ice cascades down the rock face, (which I think is mostly volcanic lava at that point, not the harder Granite you find lower), it easily smashes it to pieces. The avalanche has mass, speed and trajectory behind it, and when it gets to another rock field (which is mostly debris from previous slides, it simply pushes it much the same way a shop broom pushes dirt around. The white cloud is mostly ice.

Rocks on volcanoes fragments easily. Even if it is a relatively hard rock like granite, it is exposed to millennia of weathering where water gets into tiny cracks, freezes (expands) then when it thaws the crack is bigger so more water can get in for the next cycle. Basically, the same reason that potholes form on roads.