Since the 2.1 patch, our raid group has found that the enmity upon starting the fight with Twintania is unusual.

We are currently having our off-tank proxy-aggro the mobs while the main-tank Shield Lobs Twintania. Then, before any other threat-generation actions take place, the additional enemies are Flashed and the fight begins in earnest.

However, we are finding that the additional enemies will all attack a random party members instead of proxy-aggroing the off-tank as normal. This random party member is often a healer, but sometimes a seemingly-random DPS. This occurs despite these players being idle outside of the arena with no HoT spells and with pets on Steady.

We confirmed that this behavior does not occur if the party stays at the top of the zone near the entrance after buffing has already taken place; therefore, the enmity does not appear to be due to pre-fight buffing. However, we are not sure what is causing the behavior.

This mob behavior is not a huge deal -- the off-tank merely recovers the wayward additional enemies, and we proceed as normal -- but it is somewhat annoying. I thought I would try asking here to see if anyone has encountered the same thing and whether they have diagnosed the cause of the behavior.