Beehive basin avalanche triggered by intentional cornice drop

9500 ft E facing slope in Beehive basin.  Dug to investigate the basal facet layer on the apron, found HS 120-140 cm with rounding facets on the ground, ECT 23 on pencil firm density layer above the facets, ECTx  on basal layer.  Chose ascent route in a slope concavity to avoid potential shallow trigger points.  Cut a cornice prior to descent and struck a shallow (18 inches )  trigger point on top of some rocks which propagated about 500 ft wide across multiple ribs and 400 vert D3 R4.  Did not recognize the razor thin margin for error on this slope, scouring in the start zone which created the shallow trigger point, or the propagation potential in this zone.  

I meant to add the crown depth 18 inches to 6 feet