Can you explain this behavior?

Two of my girlfriends and I have all experienced the following bizarre behavior:

1. Girl meets guy.
2. Guy actively pursues girl.
3. Girl gets really excited about guy and is receptive to his advances.
4. Guy continues to pursue girl.
5. Mid-date or immediately after, guy announces he doesn’t really like girl.

Leading to, of course, 6. Girl is confused, surprised, and really disappointed.

Why the disconnect between what guy is saying and doing? Why would he act interested by continuing to pursue the girl and then with no warning indicate that he is not, in fact, interested?

Are we all incapable of reading signals? Are we seeing what we want to see? Are these dudes themselves confused about how they feel? Are these guys put off by the girls being really into them? Did we all make some kind of unforgivable mistake mid-date, like being horrible kissers or chewing with our mouth open or something?

What am I missing?