You didn't say how old your car is, nor for how long you've owned it, but it's probably a neglected crankcase ventilation system. Volvos typically have a "flame trap", which is a screen that allows crankcase fumes to be drawn into the intake system (the screen prevents a backfire flame front from flowing into your crankcase to ignite fumes) -- resulting in a negative pressure (slight vacuum) to exist in the crankcase. Sometimes this screen (or one of the hoses with it) becomes clogged and pressure builds up in the crankcase, and so this should be part of regular maintenance.
In other words, you car needs some until now delayed, but badly needed, otherwise routine maintenance.

But there is another possibility: at times when there is a blown head gasket (or bad rings, etc.), pressures can build up to a point when the dipstick, or seals, might blow. I hope this isn't your case.

Good luck.