It does happen. The drive home can put my partner in a bad mood, work can do it, of course having a rough moment in his other relationship can do it - the things in our lives affect our moods, and there really isn't any other way around it.
How do I deal with it? Well, it's the sullen silence that I have a hard time dealing with. I don't need details, but if it is something to do with his other relationship, I try to be supportive, and just be there for him (and FWIW, I'm Mono). We don't trash each other; we listen, maybe try to offer alternate viewpoints, and we get through it
I don't really see how rules could stop it, other than he would be in a bad mood and not be able to tell you why. If he tells you about every other bad mood, but can't tell you about this one, I suppose that's going to tell you something anyway... Elementary, my dear Watson.