Comedians seem really conscious about comedic "territory" whether it's a routine or a show so it's no surprise practically every comedian and their dog sided with Conan.Kimmel probably just realizes that the proper, cool, "hip" attitude lately is to be anti-Leno. He's jumping on the anti-Leno bandwagon, nothing more. Following the trend - the fad, as it were.
The battle lines were drawn long ago. Conan must have already had a lot of friends anyway. Somebody decided that he was to be the blameless golden boy, the put-upon oppressed victim, and that Leno was the evil dictator son-of-a-bitch. Others fell right into line after that. They're following the crowd. They're saving their own careers by picking the "right side".
Kimmel has been bashing Leno on the Stern show since he got the JKL gig, Dave has been bashing leno since they were both up for the tonight show gig. The only real "opportunist" would be Ferguson and he's pretty much said as much.