Was someone else taunting the boss? That's when I usually see this issue - someone else has decided that Puncture, Inner fire, ice staff heavy attack or the tormentor set charge attack would be a good way to play in a dungeon.

Overtaunting does not exist in this game. But a boss can only be taunted twice if done by two different persons (to avoid being able to ping-pong a boss between two players indefinitely).

If you taunt, then someone else taunts, the boss becomes immune to taunts for several seconds. If it is only you who taunts, boss stays taunted on you no matter how much you spam the taunt. So what you described is most likely the result of heavy ice staff attacks(its the most common cause) done by one of the other players.

Some bosses don't aggro even if taunted, or even if they have aggro they still go for other players from time to time. This is simply how they are coded. If I remember correctly the first boss of Elden Hollow - orc warrior Akash gra-Mal - does indeed do that, and jumps at the other players as well. So there isn't any bug there. As for aggro, there are only 3 skills that can do it:
- 1H+S: Puncture & morphs
- Undaunted: Inner Fire & morphs
- Ice staff heavy attack if Tri Focus passive is specced into.
If someone ends up stealing the aggro from the tank it's most likely the latter, as many players simply don't pay attention to the skill& passive tool tips.

The ice staff taunt isn’t widely understood for some reason. I carry a spare inferno staff on my tank to give to dps using ice staff.
The best solution, however, is to let them die.