Its mostly due to a few reasons.
1. They have not seen said individual praising the character until the action of, "Being OP," is displayed.
EX: I've always liked the Second Hokage, but seeing as he's only really in one moment in part I... not much to talk about. The War gave him more discussion outlets. However, it suddenly appears as if I am a Tobirama fan.
2. Anti-*Insert Character*
EX: An individual becomes a Hinata fan... just so they can bash Sakura, and feel they are in the, "In Crowd." Of course, this could be done for any character. Sakure > Hinata, Ino > Temari... etc. The basis is, you've joined/supported a character because it is commonly seen that this particular character, and the one you dislike commonly have opposing fan bases.
3. Bandwagoning
EX: Word for word. Bandwagoning is actually a pretty logical reaction. When a character is introduced, if you like that character, you've joined the wagon of their fan base. If a character starts taking actions you like.... naturally you are more inclined to show more support to a character.
Bandwagoning is usually held in a negative connotation, but does not always have to be. But the above are the generic reasons for being called a, "Bandwagoneer."