Instant death PvP area - what was this?

In the Borean Tundra, I stumbled into a small area where the npcs were flagged as PvP. I was killed instantly by something - don't know what, and every time I tried to rez, just being in the zone I was dead again instantly when I rezzed. I don't know what that was. Can anyone explain this?

What are these areas, who can go there and not die, etc. I think the enemies were my level.