But given the seizure that happened to the king the other Duergar didn’t assume the Paladin had attacked the King? And redouble their efforts against the party to rescue the king?
That is a good question, and I had to ponder that.
What swung the decision (and I'm sorry I didn't mention this in the OP, but I didn't want it to be too long) was that the king was an usurper, who killed the previous king and some others, and who ruled by terror. Not only was he a very dangerous warrior, he could spit brains slugs at people.
So the behavior of the paladin and how disliked the king was made the duergar hesitate for just a few key moments... enough to stop the battle. The duergars realized that the reign of terror was over.