It's the queue system throwing you into matches where someone quit. Just like in dungeons, where you get the blue portal.
Can't understand why players quit battlegrounds at all. You still get 7k AP, some gold (very little) and experience, just for coming in last.
The way I feel, if I'm putting in my time for a loss, I'm still walking away with something, even if I'm super frustrated with my teammates play or how my performance was.
But as to your topic...nothing is wrong. You're just taking up a place where someone quit, or you queued on a team that started with only 3 players and the system filled you in as the fourth.