Circle zerg (or "circle-zerg") is a term used for a defense strategy in the new Tol Barad PvP zone (although it is really just a big battleground). The basic strategy is simple: since the attackers need to cap 3 buildings near "towers" (Ironclad Garrision, Slagworks, and Warden's Vigil) to win, the defense can just get a large group to rotate through each tower area to try to take it back aka circling a zerg group.
There are counters to this tactic, but it is generally effective if both teams are relatively disorganized.