When you bought and wore your gear for the first (and only time) during playoffs.
When you can only name the BIG names on the team and half the time get their positions wrong.
When you never watch football, and suddenly when the Super Bowl comes up, you a pick team that has any kind of relation to you (like you visited the places once in your life, or has a family member who lives near there) and suddenly you act as if you're part of that team. i.e. "FRIEND" of mine suddenly saying 'Oh my God I hate the Seahawks" right before NFCCG, who grew up in NYC, and has had no connection with football up until that point and just jumped on the CK is hot therefore I'm a Niner fan bandwagon.
I know a lot of people have different definitions of bandwagon fans, hell I may even fall into some of those definitions. i.e. I really only started watching Seahawk football in 2005 the year they made it to the Super Bowl, but my reason is just. I grew up in Guam, we have no football team there, and I grew up playing basketball. I made my way out here for college and moved to Seattle in 2005 and just got into football then and adopted the team at that point. But I'm certain to a lot of fans I would be a bandwagonner because I wasn't around for the Largent and Zorn days. Hell, I was 6 when Largent ended his Seahawk career and my father being an immigrant from HKG wasn't exactly a Seahawk or Football fan to surround me with any of it.
But yeah the first three lines are my definition of Bandwagon fans as I'm definitely a newer fan and more partial to them.