I don't think you are wrong. I think that was the strongest catalyst for Derek; I also think that he was affected by the green-eyed monster when he saw the pregnant woman's husband talking to Addison, and her being fully engaged (didn't he also know that that was the same man that told Addison that she looked like Catherine Deneuve?). I want to say that there was also something else, but I can't remember what it was and I have erased the episode.
Finally! Someone who acknowledges their part in relationship failure! Being ignored will chase somebody into the path of another very quickly.