i don't know if this is a new game mode or something, but i've had three missions in a row now where my dudes end up slaying more than twelve aliens. and this is on a new game.
the first one was ridiculous, I killed 19 aliens and more kept spawning right next to my guys who were cornered up and had 360 degree security. then the mission ended, which was a mixed blessing since my snipers had racked up huge kills, but both ended up dying from mele aliens appearing behind them from a dead-end that I had already cleared three times before.
i don't know if this is supposed to happen like a kobaiashi maru scenario, or if this is a bug.