What limits most integrated circuits to 150°C junction temperature?
The materials that the transistor is made out of primarily defines this.  Germanium junctions will actually be much lower, more like 70C.  Silicon and Gallium-Arsenide can handle 150-200C.

Someone with more semiconductor manufacturing experience can probably give a better answer but I believe that most of these junction limits are imposed due to the transistor crossing a thermal runaway point.  Both BJTs and FETs, although for different reasons, have the characteristic that as temperature increases so does power dissipation, which of course creates more heat.  As such once you break a threshold, the junction temperature will will rise very fast to the thermal breakdown point of the materials that make up the junction.  Which if this occurs for very long results in smoke, and lots of fun smells.  

I would venture a guess that 150C in the junction is roughly where normal packaging materials / the junction materials themselves can no longer pull enough heat out of the junction to keep it out of runaway and that the semiconductor materials themselves start to incur physical damage shortly after this temperature.  

I don't exactly know what this process is, but it may simply be the point at which the crystalline structure begins to break down.  A silicon transistor is normally made with a silicon crystal thats doped with boron (p type) or phosphorus(n type).  You may get a more exact answer if you look into the chemistry of those structures.