Blowby gases are recirculated into the intake in an effort to burn all the fuel going into the engine. Blowby gas usually carries an oil mist with it, although this tends to separate out in the valve cover baffle.
Problem is, over time the inside of the CCV line picks up a layer of oil sludge, and as this gets longer, you'll start seeing it in the air filter.
The fix? Clean it. Take the line off and blow it out with carburettor cleaner - and maybe clean the baffle in the valve cover right before your next oil change.
This is a common issue, and also (usually) fairly easy to fix.
5-90