This is most often due to the engine oil level higher than it should be. With Briggs engines, the oil level should be checked while the machine is sitting on level ground. The dipstick should be pulled and wiped clean, and then reinserted and screwed down completely and then unscrew it again to read the oil level.
If the dipstick is only pushed down against the top of the tube without screwing it down, the oil level you set based on that reading will be somewhere between 1/2 quart to 1 quart over the correct fill level.
Another thing that may cause this is due to how the tractor is driven in conjunction with a slope. If the tractor is being driven fast into a turn that includes some sloping terrain, the combined slope along with the sideways force imparted by the turning action can make the oil in the sump run to the outside of the turn (inside the sump) and climb up the inside of the crankcase walls. If the oil mass moves high enough for the crankshaft to strike the oil, it will make a huge splash, sending oil droplets directly against the crankcase breather. Some of that oil will find its way back into the intake and get fed to the engine.

Plus ensure that there is no fuel in the oil as well.