Pennards since 2007
In 2007 there were 2 groups that camped together from the eFestivals Forums:
Camp Triangle was so named because of an area on the 2007 Glastonbury Site Map on Pennards Hill which is in the shape of a triangle. It's as simple as that.
This could, of course, be entirely wrong. You'd have to ask Chocky or Paul™, for the real answer.