I am currently playing Overrun and I can't seem to figure out when the mercy rule applies. As some matches finish due to the mercy rule in the first section but others do not. Is it time related or score related or something else ?
The Mercy rule only occurs in the 2nd round of Overrun. It takes places when the 2nd attacking team makes it further than the 1st team did, and there is no point of the match continuing (since they've already one).
For example, let's say you're playing on a map with 3 points to destroy. If the 1st attacking team only destroys 1, then AS SOON as the 2nd attacking team destroys 2 objectives, the match is over and the 2nd attacking team wins.