I found a saved game from a few turns before the event, so I ran some experiments.
There is no option for me to accept capitulation from Louis; it is red on the trading screen.
- If I continued doing what I had done in the original playthrough, the same thing happened: Louis capped to Monty even though he was only at war with me and with Toku.
- If I bribed Toku into peace with Louis: Same thing...Louis caps to Monty
- If I use Worldbuilder to give myself troops and take one more of Louis' cities before ending the turn, taking one more city from Louis makes him offer capitulation to me. So, he's close to it already.
- If I use Worldbuilder to replace several of Monty's core cities with ocean
, then Louis doesn't capitulate to anyone.
- If I use Worldbuilder to add about 20 population to each of Sulieman's cities--thus putting him way ahead in score--then Louis still caps to Monty. Same thing if I also give Suleiman lots of Modern Armor so he's way ahead in power too.
Conclusion: I'm guessing there must be some random check made with each leader, and Louis somehow decides to cap to Monty even though they're not at war. While the other leaders' power and score don't matter, Monty has to be high enough so that Louis will consider it.
Seems like an un-controllable situation. It might happen any time you're on a rampage.