If you've ever tipped the push mower over to clean the underneath of the mower it can result in oil draining out of your lawn mowers engine and into the carburetor which causes the air filter to become saturated with engine oil.
If that's not it it could be a problem with the engine causing oil to bypass the engines piston rings and then getting into the carburetor.
Most common cause though is the engine being tilted when working underneath the mower if it's a push mower.
If it's a riding mower then the same could happen if you happen to lift the riding mower up and then cause the engine to tilt enough to cause engine oil to drain into the carburetor which then gets into the air filter.
Or it could also be an internal engine problem or you could have overfilled the lawn mowers engine with too much oil.