I've been playing this game for a long time, and I have never ever seen the United Planets come up. Ever.
The only thing I can think is that it comes about when you have found all of the races. I always play slow games on the largest maps, and I often don't find all the factions until late in the game. Many of my games crash before I find everyone (yay game!). However, I have finished the game before, and I've found all the opponents before. Is my game on crack?