What is the current theory about the formation of Tibet?
Per yesterday's What-If,


  If you make a building too big, the top part is heavy and it squishes the bottom part.


In the context of Tibet, what this means is that rock has a yield strength: pile it up high enough and it will break (via earthquakes); and if you make a big enough pile, especially of continental material, then the concentrations of radiogenic isotopes will be high enough to significantly raise the temperature - and lower the strength - of the rock in question.

In the case of Tibet and the Himalaya, height increase is due to north-south shortening as the Indian plate impacts the Eurasian plate, i.e. the Tibetan plateau initially formed via exactly the same process as the Himalaya. In order to get an approximately stable height, this increase has to be balanced by decrease. The principle processes causing decrease in height in Tibet are:


east-west extension (see. e.g. Dewey et al. (1988), Tapponnier et al. (1986)), typically along strike-slip faults: this is clearly visible not only in geological maps but also in aerial/satellite photography
erosion rates increase as topography becomes higher, especially in areas of active mountain-building, in part through a change in which erosion processes are favoured


That the Tibetan plateau is of such uniform height corresponds neatly to calculations suggesting that this is the maximum possible height for such features if they are of typical crustal composition.