Have a look at the scoreboard. It's usually because the teams are uneven. There may be a few players AFK or tanks/recons camping from the spawn. I've noticed that in Conquest, the game will become v. unbalanced if there's a difference of 1 player eg 32-31.
If everyone was not shooting each other, then the team with the most number of players would win automatically by capping the flags and having greater ticket bleed.
With regard to shooting, the bigger team can cover more angles, so not only do they have an advantage in capping flags, they also have an advantage in spotting and shooting the other team.
If there's a huge skill discrepancy or cheating going on, then players tend to leave the server, which makes the problem worse.
It's the devs' fault for allowing games to start with unbalanced teams. It's been like this for years. In older games, where you could rent your own servers, the admins could run team balancing scripts which annoyed the person being moved to the other team but at least it prevented complete stomps.
