Trees, bushes or rocks protruding though the slab that may help hold it in place.
Anchors need to be thick enough to be effective. The more thickly spaced, the more effective. Sparse anchors, especially combined with a soft slab, have very little effect.
Anchors that don't stick up through the weak-layer have no effect. They need penetrate to well into the slab.
Spruce and fir trees with branches frozen into the slab are a much more effective anchor than a tree with few low branches such as an aspen or lodgepole pine. Also, snow falling off of trees tend to stabilize the snowpack around trees.
Anchors hold hard slabs in place much better than soft slabs--like the difference between cardboard and tissue paper when affixing them to a bulletin board with a thumbtack.
STRESS CONCENTRATION POINTS:
Avalanche fracture lines tend to run from anchor to anchor because they are stress concentration points. In other words, you stand a better chance of staying on the good side of a fracture line by standing above a tree instead of below.
"Flagged trees" -- trees with all the uphill branches stripped off--mean that tree regularly gets hit by avalanches.