The firefighter may have been spraying the foam before they extracted the pilot. That would make sense as they would not want to start working the aircraft to extract him without laying a blanket of foam around it to stop the whole area catching fire - those crops look pretty dry.
Secondly, AFFF forms a barrier between the fuel and the environment. It will seal in fuel and vapour and they will spray a blanket of AFFF to stop the environment being further polluted until the Environment Agency arrive and can make an assessment. The recovery of crashed aircraft cannot begin until the UK AAIB arrive and so the firefighters must also continue to protect the evidence until they arrive or tell them that the wreckage can be moved.
Complex stuff is emergency response - it’s not all about the first 5-10 minutes.