I don't really know anything about seismology but I'd bet it has to do with the same reason the Himalayas are there
https://en.m.wikipedia.org/wiki/Main_Himalayan_Thrust
https://en.m.wikipedia.org/wiki/Main_Central_Thrust
https://en.m.wikipedia.org/wiki/Main_Frontal_Thrust
Basically the Indian tectonic plate is still pushing into the Eurasian one, which is how the Himalayas came to be, resulting in major seismic activity in the area, such as this:
https://en.m.wikipedia.org/wiki/1950_Assam%E2%80%93Tibet_earthquake
The 1950 Assam–Tibet earthquake, also known as the Assam earthquake, occurred on 15 August and had a moment magnitude of 8.7. The epicentre was located in the Mishmi Hills. It is one of the strongest earthquakes ever recorded on land.