AI resigns without fighting in King of the Hill?!

So I set up a 12 player King of the Hill match with 3 teams (4 on each team) against Hard AI (Attacker).

While the game went normally for 15 mins (AIs did fight in the centre for the Hill), but all of a sudden all AIs resign and I get a victory for no reason. Not just 1 team, both the opposite teams resigned at the exact same moment.

I saw the replay, there were no armies attacking the enemy base, they should have no reason to resign.