This slide was already reported to Craig Gordon with a nice video by the party involved. The area is east facing with many acres of flat terrain where the winds can grab lots of snow on deposit it onto this east facing slope. The snow pack in this area is typically thinner and weaker the further east you go from the Mirror Lake Highway.

This hard slab avalanche was possibly triggered lower on the slope where the slab was thinner. Once the weak snow collapsed, it pulled-out into the deeper part of the slab just off the ridgeline.

I observed this other slide just to the south of the bigger avalanche on a similar aspect.

The general snow pack set-up in the Elizabeth Pass area, 10,000' in elevation.