Nepal’s Deadly Flash Flood: What Triggered It?
https://www.youtube.com/watch?v=9ZvUQ7aoehw
Speculatively, the ice and everything in its path crumbles as well as melting effects all collide
at the base at the river valley.
This causes the water to back up behind it all
while the mountain of ice and debris follow the
next logical path downward as its only route.
The backed up water follows behind and eventually adds forced momentum.
Raging is an understatement compared to some of those scenes.
That's my doofis assessment but may help a bit.