In a confrontation between two parties, it's more likely that the stronger one will pose the greater threat to you. By supporting the underdog and hoping for a fluke victory, you're increasing your own survival odds. It seems we're probably evolved to seek parity -- where we then have the best chance of dominating -- instead of seeking dominant leaders and siding with them, which is a far more complex and less certain process.

Am I missing something? Also, it would be interesting to see whether females and males have the same reactions toward the overdog.