Yep I noticed that also in my game, just always forget to say something about it on the boards. It is indeed from the malevolent ideology trait that gives you a permanent boost to morale for every planet you invade.
This trait also makes it so that malevolent civs never really have to worry about the Large Empire Penalty to morale. You get +1 to morale for every planet you conquer, and the LEP is only -.2 for every planet you own. The longer you go conquering, the less need you have for wasting space on morale boosters for your planets. Makes it pretty easy to keep morale on your planets capped at 100%; which seems kinda ironic that it is easier to keep the populace happy as "evil" than it is as "good". Neener neener to all the "goody goody" civs 