I think the farmer caught up with one of the group leaders and tore a strip off him.
We also get scout groups, schools etc turning up without informing anybody because they've always done that. I ask any I see to contact the Estate Office with risk assessment/insurance details. If an incident occured without permission, I'm not sure how the landowner/farmer would stand.
If your interested see following (and subsequent) article - all are free access:
http://bowjibushcamping.co.uk/wp-content/uploads/Running-a-bushcraft-school-H-S-Legal-summary.pdf