I like Goodale's suggestion. Hornblower or Aubrey would have set them straight. This is the worst desecration I have seen. Hopefully the did leave a calling card or partially eaten food, garbage or prints on the rakes for DNA.
I wouldn't look at juvenile delinquents - combination of too much physical labor and too long to keep their attention (unless it mostly a sports team).
I would have considered member's kids, but the members' age would have to be below 45.
Nor at disgruntled employees, unless they new exactly where to dig.
More anarchistic than eco-terrorism.