If all the timelines are converging/disappearing/being culled into one true immutable timeline, I wonder if the sheer enormity of how much better things could be is what drives Nozdormu to break from his duty.
He sees an opportunity to establish the perfect future from the convergence of an infinite number of alternate pasts, and takes advantage of it, splitting himself among the timelines as infinite versions of Murozond. All of whom are shaping their timelines to fit some grand design, in opposition to the Bronze flight.