What's momentum?

Momentum in a political campaign is such an ephemeral and difficult to rationalize thing. It seems to actually exist, but why does it happen?
There are obviously the practical advantages of racking up endorsements-influential email lists, superdelegates, etc.
But this post by Ann Althouse hints at something that's been in the back of my mind over the last month or so. Is it possible that momentum is partly the result of people feeling that they have permission to vote against the front-runner only after (s)he is wounded? Obviously that makes some sense with politicians' endorsements-they simply can't risk being on the losing team because they need the access to the winner their support can buy.
But with ordinary voters, is it possible that once the "inevitable," powerful front-runner is shown to be vulnerable, people feel like they are then allowed to vote some someone else? I'm not sure I like the idea, but it does make some sense from an evolutionary psychology perspective.
I should also note that I ordinarily don't agree with a lot of what Professor Althouse posts about politics, but this particular post got me thinking.