Internal combustion engines mix fuel with air, burn it, and the increase in pressure is used to force down pistons, etc. etc. Some gases slip past the piston rings into the sump. On earlier cars this pressure was released by a vertical vent pipe topped with a canister full of steel wool; worked well. Recently governments required cars to clean up their atmospheric emissions by reburning sump gases in the engine. This has led to all sorts of problems.
On your 2.0 the cam cover space is connected to the sump by the camshaft oil drain and the pipe on the passenger side. It should always be maintained below atmospheric pressure by the breather system, sadly not always the case as you observe. At idle, the small breather pipe, connected to a small hole between throttle and inlet manifold, does the job; at other engine speeds the large breather pipe, connected between air cleaner and throttle, does it.
Then the car gets old, pipes get bunged up, the pressure rises under the cam cover, oil is blown out, and we wonder why.