The strafing attacks were both random and planned. When escort fighters broke escort (like at limit of range for a Penetration escort, one or more flights might drop to the deck and attack a sighted airfield on the way home.
A pair of fighters chasing a 109 might spot an airfield and shoot it up. Notable was a planned strafing attack b VIII FC on April 5, 1944. The 4th FG destroyed 45 on airfields around Berlin and the 355th destroyed 46 around Munich, including 6 air.
The fighter groups were encouraged to strafe targets of opportunity on the way home after escort.