Re: Rooting for the front runner...
To answer the OP's question, I believe that that it is a combination of - not to sound like a curmudgeon - the short attention span of people, particularly youngsters, due to today's technology/ability to view virtually any game/team.
Granted, one can argue that bandwagon fans on the Pro level have been around forever, especially among fans not living near a Pro City.
Examples: Raiders, Steelers, and Cowboy fans in the 70's. 49er fans in the 80's.
The major difference is that most, if not all of the aforementioned fans remain fans of said teams today.
"I have just been more than a little suspect of all the trades since the Willy (Scott Williamson) cash grab. That one left such a bad taste in my mouth that even a 1985 Dom Pérignon couldn't cleanse it." -- Creek14