Why would a lawnmower engine leak oil out of the carb into the airfilter?

can anybody help me with this? i found the lawnmower out in the trash got it to work but then found this problem later the mower still runs fine except for this problem i whiped down the filter compartment and the engine and couldnt get it to leak again. this was when i had tipped the lawnmower before i was about to do an oil change on it but i had not yet touched any parts and the oil cap which is located on top next to the gas cap was on tight and then i checked the drainplug located under the mowing deck to see if it was tight and it was also. this is a briggs and stratton 6.5 hp engine on a craftsman lawnmower.