perhaps he went because he wanted to see you? that would be my guess. and perhaps he probably wanted you to see he wasn't drunk.
all that aside though, it's not fair play to sabatage you like that. he could have told you that he planned on attending. i doubt that it slipped his mind, it would appear to be a deliberate ommission for whatever his secret agenda is.
i wouldn't spend much time trying to figure it out; it's just another crummy inconsiderate thing to add to the bonfire.