I guess it happens only in Survival mode
When the round is over and the game is paused (very common situation in Survival CW's) a player joining the game (some team, doesn't work with spectators) causes one of the teams to get a point.
Example: It is 4-1 for alpha, the round is over, the game has been paused. One of the players needs to rejoin, so he leaves the server and joins it back. While the game is still in pause, alpha gets a point when the players joins the team, so it's 5-1. No need to mention how many issues it causes in Clan Wars...
It's been noticed that this bug can be reproduced for sure only when the round is over and the players haven't been killed yet (there is some time between the team getting a point for won round and all players from that team getting killed to start a new round).