I don't know what volvo motor you have, but we have a Volvo D-6 Diesel in our work boat. There is a filter in the crankcase breather hose before it get's recycled into the air inlet. I suppose if that filter were to plug up it could develop some crankcase pressure.
The system you had just described is the one I had with my old Volvo MD7A. This motor (Volvo 2002 is the model number) has a "closed circuit crankcase ventilation" which works in a way I have not yet figured out.