This happened a few weeks ago.
I went through a portal to enter the part of the level that contained Tahazzar and his gang. As most of my party had ranged weapons, I had those party members take some shorts at members of Tahazzar's gang who were within range, before quickly returning back through the portal I had come through. I saved my game and then went back through the portal to return to Tahazzar and his gang, intending to do same thing.
But when I returned to the level, all the game displays suddenly disappeared, and then things really got strange. Because the game took control of my party and started attacking the members of Tahazzar's gang. My party attacked one enemy, killed them, and then moved on to the next one, my party going through Tahazzar's gang in an anti-clockwise pattern. As this was all going on, Tahazzar and the remaining members of his gang just stood there, doing nothing.
The only member of Tahazzar's group that my party did not attack was the Succubus; once they got to her, the game displays reappeared and the party was now back under my control.
It was most bizarre.
Has this happened to anyone else? What causes this to happen?