Wow. crazy. Wonder if while they were up there they dislodged something to trigger it. Also if anyone else was at the lake , how inconsiderate his yelling was. He probably would of gotten one of my used socks.
My brother and I saw something similar in the Absaroka/Beartooths. Super heavy rain, then sounded like a jetliner was flying in the valley. Then a debris flow of car+ sized boulders ripped out of a couloir and flowed for about 10 minutes. We were speechless.