In any competitive game, if you’re ranked as #2 player, but you can join forces with player ranked #3 to eliminate player ranked #1, your best course of action is do that.

After player #1 is defeated, you have a good chance to defeat player #3 (because you’re #2, meaning you’re better player).

Player #3 joins this campaign in hope to defeat you, but also because they can get whatever second place gives.