You can see my extended explanation above, but in short it was due to a glacial moraine outburst flood upstream, releasing two-thirds of the water of Lhonak Lake in one go. The lake was known to be at high risk of bursting the moraine.

The lake itself was caused by glacial melt due to global warming, first appearing in the 1960s. The bursting of the lake this summer will almost certainly be traced to the record high temperatures recorded this summer, which led to an increase in size of the lake and overtopping the moraine that held it in place.

This article has satellite images showing the recent growth of the lake and then the recent release of most of its waters:

https://www.ndtv.com/india-news/sikkim-flash-floods-lhonak-lake-isros-satellite-images-show-how-sikkims-lhonak-lake-burst-caused-floods-4450175