It seems that the aliens in campaign missions always target the unit with the lowest health. This makes for some interesting strategies, I've noticed that if I leave one unit with low health at the back of the map most of my units can avoid alien weapons fire as they all target the said unit.
I had a unit about 4-5 spaces away from an alien coming out of a door and instead of targeting him he fired on my injured unit clear on the other side of the map.
Part of the time this is very annoying, and makes you have to protect or hide your injured units. But most of the time its a very exploitable perk. I encountered the same thing in previous versions.
