Good catch. I noticed the bug several times before, but always forgot to look into it.
I have been able to cross-match the screenshot time with the combat log, and the debug log, and the bug will be fixed next reboot.
Basically what happened was that after two minutes in the ice realm, Frigus would reengage and track down players. However, this timer wasn't cancelled if you reached him before the two minutes were up, so if you - by chance - were down there the second/third time exactly when the timer expired, he'd come after you (if the timer expired early, it'd just do nothing, because he couldn't target anyone, if the timer expired later or when you were up with him, you'd not notice, because he'd be fighting you anyway).
I removed the timer, because there is no need for it. If a group stays down there for too long, the increasing stacks from Grim Cold would kill them - and if that doesn't do it, Frigus would re-engage them once the berserk timer is up.
