Like anything mechanical, your engine wears as you put on the mileages. When the car was brand new all the tolerances were tight, but as the engine wear the tolerances become more loose. Rings in the engine starts to wear due to the constant up and down movement of the pistons so as the rings and cylinder walls wear, more combustion gas is able to make it way into the crankcase. The combustion gas mixes with the oil in the crank case creating a mist that is drawn up by the CCV system back into the intake to be burned.
Also short drives on a car is a lot harder on the engine since the engine doesn't have time to get hot enough to burn off any water vapor that may have been created. The yellow mayo is when you mix oil and water, but to get rid of the water in the oil, the oil has to reach a temperature close to 212 for the water to turn to a vapor. If you are following the BMW recommendation for oil change every 15,000 miles perhaps you should perform the oil change more often say 7500 miles or 5000 miles.