#7 - Oct. 18, 2017, 4:28 p.m.
If you are PvP-flagged and relocate to friendly territory (such as once your main cities) - then you will remain flagged for 5 minutes during which you are still vulnerable.
This would mean that a person who is out in neutral territory ports to their city, they would be vulnerable after arriving if a sneaky sneaker was waiting for them.
If folks are porting into a city and are already un-flagged for PvP, it means they were already not-flagged - such as by being in Dalaran.
Most players in a major city are likely to not be PvP-enabled so they can safely use the auction house or relax, but the NPCs and faction leaders are always eager to pick a fight!