Natural wind slab. The Wellsvilles did not receive as much snow as the Northern Bear River range but they did get hammered by strong winds.
From the Flygares;
"Lots of activity on the Wellsviles. There is a fairly decent sized slab at the head of Shumway canyon maybe 50 feet wide and ran 8000 probably only a foot deep. Lots of debris at the head of Pine canyon looks like cornice falls started the slides. Many other cornice fall avalanches along the range but no more significant slabs."