It's from the plot of Avengers 2 and Vision's MCU origins. Vision was originally being created by Ultron to create a perfect body for himself (through the combination of vibranium with organic tissue). So this What If scenario would just be that the Avengers failed to stop his body transfer as they did in the movie.
A popular theory I'm seeing is that this particular Ultron is also a multiversal traveler, which is why his sudden appearance seemed to surprise the Watcher. Could be setting up an overarching plotline.