Northumbria Police says "enforcement action" was required at the demonstration.
An investigation has been launched and police are working with the Health and Safety Executive.
Police are keeping an open mind as to the motive but said the assault came shortly after a dispute.
Nine-year-old Sophie has collected more than 370 bags of rubbish in a bid to clean up her town.
Malcolm Flynn, 72, died at the scene after being charged by cows near Thirlwall Castle.