When your some ones best friend for a large amount of time, then after they lose all their good/best friends they cling to you. Then around 6+ months later when he/she gets their friends back, they start pushing you away slowly and moving towards their old friends again, but still call you their best friend.
Situation is made worse when said person is your Ex and you have feelings for them.
Bob: wow man did you see how Carly totally made Alex a Backseat Best friend? she completely ditched him.
Jake: Yeah man, really f'ed up