Fuel getting into the crankcase requires a couple of different problems to add up. A float float valve or the float itself could be stuck in the open position, or the valve needle isn't seating properly. For this to cause a problem, either the petcock would have to be left on, or turned off but faulty.
Ordinarily, this would only cause fuel to drain out of your float bowl overflow tubes, but if one or more of them is clogged, the only place for the fuel to go is through the intake ports and down into the cylinders and crankcase, mixing with the oil.
