It would be the (vent) pressure inside the tank forcing oil into the oil passages. So the pressure inside the passages will be the same as the tank pressure.
Just for example purposes lets say the tank built up 10psi of pressure from the crankcase vent. That 10psi forces the oil into the passageways.... the passageways are pressurized to 10psi. But..... that same 10psi is inside the crankcase because they are linked through the breather tube. Inside the crankcase happens to be the outlet of the oil passages. There's zero pressure differential... no oil will flow until the tank fills up with oil and the pump pressure can exert a force greater than what's in the crankcase.