Basic physics and a common side effect of the Omega design...
Hot oil fumes get sucked up through the breathers... Some goes straight into the plenum via the top hat fitting on the breather bridge (the two small pipes one is the breather box vent, the other is actually from the purgevalve/charcoal box). The two larger pipes, (from the large outlet on the breather box) vent to the outside of the throttle body... When the throttle is opened, the airflow creates a hefty vacuum on the breather system.
This draws up the hot oil fumes from the crank case. As this cools it condenses back to liquid oil and will effectively drain in to the mouth of the throttle body and on to the intake pipes. After a while this adds up to a reasonable amount of oil at the lowest point.
Incidentally if there's any condensation within the engine, this becomes steam along with the oil fumes, and as the engine cools, this also condenses, usually within the breather bridge. This is what clogs it all up and why regular breather cleaning and decent oil are important. The small pipes clog first, which increases the apparent amount of oil at the intake.
Better oil, more frequent cleaning and less short, cold journeys are the easiest ways to counter it, but you can never stop it happening.