Check the tappet valve clearance, it can happen because the exhaust valve remains open which can increase pressure in the crank case forcing oil out through breather pipe. That just happened to my geny, I just noticed few days back but I was too late, the damage as already been done, as its oil switch is disconnected and it continued to run on low oil levels. Planning to open it up n repair it in a few days.
It can also be due to worn out valves, pistons rings. Take out the spark plug and see if there are any signs of engine oil (burnt or otherwise) on the plug.