I looked around the forum and couldn't find any mention of this, but I'd like to give my theory on what triggers the Marius Reforms.

I got mine around 230 B.C., the exact same year I managed to build an Imperial Palace. I was intrigued and went to a previous saved game where I had the Imperial Palace still qeued up, and used the "process_cq Carthage" cheat, and like magic, the Marius Reforms happened.