I have broken 1 birf (treated longfield) and 1 stock short side inner, both on the passenger side.  I have no idea how the birf broke, wasn't doing anything hard it just started popping.  The short side inner broke when my wheels were straight.  but that was becasue the passenger side tire was wedged against a rock on 3 sides.  It all depends on what tire is bound up when it breaks.  If you bound up the left tire then that axle/birf will blow, same thing with the other side.  I think the reason more people blow the short side is because they can't see what that tire is doing unlike the drivers side.  I couldn't see my passenger tire caught up in the rocks when mine went.  but if i see my drivers tire get caught up then i back off.