usually a window will break beacause of imperfections durring the window-making process, stress spots, weak spots, spots with minerals ect ect. and the vibration just hits the resonant frequency of the glass, or just vibrates it as you hit a bump, and the glass gives way.
other times, its because there may be a problem with the window frame 'pinching' or putting stress on the window because it was not put in perfectly (not all cars are perfect, there are defects) and you cant even tell. then - like before, bump+vibration+metal expanding\contrating\torquing will crack the glass.
maybe there was a physical chip in the glass too.
99% of the time, the bass is only a added stress to the problem, not the cause.