Clarification of Camp rules

[Splorf22]

So I have gotten into this situation several times over the past weeks. I arrive at camp x: (necro-guardian in sebilis, verix in KC). Mobs are up. I begin to slaughter looking for my resist gems/keypiece.

Group arrives. They wiped/evaced/were otherwise not there, but were there at some point in the recent past. Confrontation ensues and I'm given various lines like "NG is not a solo camp, I've talked to GMs" or "We have 15 minutes to reclaim our camp after an evac for a pathing bug" or "We're not giving our camp up to you because we were trained" (this from the KC basement group. How the heck do you train yourself in the KC basement?).

At this point I grumble a bit and leave, although I could probably out-dps some of these groups with a hasted DW pet and worst case get 50% of the kills with memblur, because I can't be bothered to get into huge confrontations over pixels. But it's starting to happen a lot and it's beginning to irritate me a little. Soon I'm hoping to have enough resist gems/keypieces to move on from these popular camps but in the meantime, I'd like to know exactly what the rules are: are there any extenuating circumstances which would give a group that wasn't there the right to the camp? I know server reset counts but I'm not aware of others.