Myrmidon was hunting Imps in Stygia 20 minutes ago, when the last one somehow triggered a zerg flag. Didn't notice at first; just thought RNG got disagreeable. No alts were near. Went back in the browser to confirm it was the attack on the last imp that triggered it.
All I can think of is my other Myrmidon was hunting at the other portal shortly before. Maybe an Imp he killed got respawned and wandered over?