I've seen the same sort of thing in pre-industrial-era Japanese house design. My guess, is that although the roof is fixed down, it helps to have it also weighted down with big rocks when the wind picks up. Japan gets typhoons, and I think Tibet is kinda windy on account of being high up.

Right, I had always assumed that it was to keep the roof on. For the reply below, temperatures vary wildly, as it's in altitude - even between the shade and sun in summer, there's a large variation, and in winter, it gets coooold.