THAT'S why it was like that.
When I hear "flash flood" I think of massive amounts of rain. But this started as an avalanche sliding into a lake basically emptying the lake that then followed the creek/river path to obliterate everything.
When we talk about glaciers melting, it's always accompanied by a visual of a big chunk of ice falling into the ocean.
I've never seen it shown as a glacier go down as an avalanche into a lake.