Maybe it is caused by the Lunatic interactions? (I can't remember the exact name of it, but it had Lunatic in it) They happen only during full moons, and they can happen to any sim. They are autonomous as well. There are several different ones. I had heard about one where the sim will actually go up and kiss any sim, regardless of gender preference or if they were married or not. I couldn't stand the thought of a dumb interaction messing up my Sims' relationships, with no way to stop it. (It's like the scolding interactions) So, I have added to my mods collection. lol