This is wet sumping. The engine vents through a hole at the back of the crankcase which is drilled into the end of the camshaft bush housing. If the bike has been standing for a long time, the oil collects in the crankcases. When you start the bike there is more oil in the crankcases than the oil pump can cope with so the timing gears and crankshaft fling the surplus oil out through the breather and a large puddle will collect under your bike. If you check the oil return in the oil tank you will find that, initially, the oil returns as a constant flow, but as the pump evacuates the sump there will be bubbles in the oil return. At this point, the crankcases are empty, and the passing of oil through the vent should cease. This is a common problem which most people overcome by starting their bike maybe once a fortnight, or by removing the drain plug (if you have one) from an aftermarket sump plate and allowing the surplus oil to drain out.
