As for how such a large population got to Tibet - and since this is After 1900 - I'd say a population resettlement would be the best bet on how. I mean, there's just too much overpopulation in eastern China, and moving people to less populated places might be a good way to make more room, and to make the citizens that previously had nothing to do something to do. I'd bet Mao would think of something like this and move hundreds of thousands of people (even millions!) into sparsely populated areas of China's wild west.
Of course, moving these people to Tibet would not make much since. The whole purpose of a transmigration program is so that people with nowhere to go and nothing to do could move and be given a job. Most of these occupations would be as farmers, or herders (as can be seen with previous transmigration policies, such as in America during the mid 1800's and Indonesia in the 70's). Tibet has little land for these agricultural jobs.
But, for the hell of it, lets say that Mao decides to move people there anyway - he's done crazier stuff. Most of these people would probably be the poorest of the poor, beggars, petty thieves, laborers, and the like. These city folks would be taken to a new land to start up on farming or something, unfortunately they have little experience. The crops would probably die, and these poor guys would probably die of starvation. The little crops that are grown would either be eaten by the farmers, given (or taken) to/by the Chinese military and government. The rest would be stolen due to probable famine.
Then, there's the problem of ethnic conflict. America had this with the Native Americans, Indonesia with the Batak tribesmen. The transmigrates in Tibet would probably have to deal with the Tibetians, who would be pissed that their land was given to some slumdogs from the east. They would fight back, and an insurgency would grow (not sure about this area of the world at the time). The transmigrates would probably fight back, flee, or request military help.
Basically, if there was a large population by transmigration program, Tibet becomes a (more) violent and war-ravaged place