I am not much help on this problem, but all i can figure is there is pressure building inside the motor it could be from weak piston rings but if that was the case it probably would not run very well. it is possible the crank seal is leaking allowing cumbustion gases to pass into the transmission case pushing your oil out your crank case vent.
as for the two previous replies they are mistaken because there is a crank case vent tube that is located about half way up the radiator on the right side of the bike