I think this is right. We don't notice when things run smoothly, only when they go awry. Of course, if you want dramatic political change, "running smoothly" isn't what you're looking for. This nomination was unusual because: a) the nominee in this case wasn't the President or VP, which usually dampens any rival campaign before it gets started; b) the nominee was nonetheless more or less widely regarded as "next in line" in a way usually reserved for the VP; c) the nominee had unusually high negatives; and d) an Independent 'crashed' the Democratic party, bringing a lot of non-party voters with no vested interest in the existing party hierarchy. Had any one of these conditions been different, I don't think we would've seen the same degree of discord.