“I wonder if construction activity was the cause of the avalanche.”
According to the report I saw, a huge chunk of a glacier broke off and blocked a river. A significant amount of water built up behind the blockage. When the blockage broke days of ordinary flow came down the narrow valley in minutes.
Probably, in a Western culture the possibility would have been identified, and some monitoring would have been set up. For example, the flow of the river would have stopped, and someone would be detailed to go find out why. But this place is dirt poor and just isn’t set up to operate for maximum safety.
That was the mechanism of the vast Missoula Floods 10,000 to 12,000 years ago. Huge ice dams formed enormous lakes. Dams failed and massive amounts of water flowed out. The process repeated many times until earth warmed up. You still have huge parts of Idaho and eastern Washington state with little topsoil.
For how long was the water blocked by the landslide debris? Or how large was this new lake, how long did it fill up? It seems like a tremendous amount of water. This didn’t happen overnight as some people are saying, it doesn’t seem to me. Could be wrong, often am.