AI turn CTD's are really difficult to pin down. ALmost impossible unless they happen directly when a real player does something that causes it and reports the problem, then by getting rid of it we can be sure it won't happen during AI turns. It could be connected to something you were building or recruiting, or it could have to do with a faction being destroyed, some reform error or trait error elsewhere, a building error somewhere, etc. Having a backup save from a few turns earlier would be terrific, then maybe you could avoid it.

Playing a campaign, in any sort of beta or buggy full release, I always try to keep more than one save game, switching off every few turns, to keep from having campaign ending problems like this.