Today I saw a thing I never saw before.

In a game I have UE as only friend, on a turn I had a message saying IIRC that a friend company would be dead in 5 turn. So I watched and saw that a friend system has been invaded by Pirates ! I further send a scout on the system and saw a pirate fleet with UE design ship on it.

Is it the result of some system rebellion (only thing I can think of) ?

I guess someone did use mercenaries in this case (you find tech in military tech tree).