I've seen it happen many times to my dismay. I play with custom AI empires and I always hate to see the Hegemonies and Democracies randomly decide to reform to MegaCorps, as it ruins the RP and backstory I've come up with them.
In fact, I've been experimenting, without success, to prevent the AI from reforming its government authority. If anyone know how to forbid the AI to do so, kindly let me know please.