I figured out a solution to a problem many posters in other threads are having.
They want to be able to change a Core planet to a Colony.
You did not want a Core planet to become a colony.
The solution for those who want what happened to you but can not get it to happen is to not want it to happen and it will.
Joking aside I think this is either a random bug or else the planet is or has been under an attack either physical, or influence centered.
I think you would have noticed the first.
The second may have skipped by you because of the horrendous notification interface I mentioned upthread.
May have been another form of no notice notice in which the Governor defected.
Otherwise I can think of no other reason for this behavior.
This is however one of the reasons we were given access to the game this early, to give them more eyes to find glitches so the can iron them out before release.
A question just so people do not get caught assuming.
The planet still belongs to you?