I always imagined the horde an alliance, humans and orcs more specifically, as the Soviet Union and the United States. Both of them worked together to defeat the greater threat, Nazi Germany. But as soon as that conflict was over tensions between the two rose greatly simply because every single ideal each nation held was the total opposite of the others. There was no way there was ever going to be peace between the two until one of them was gone. Which is just how the humans and orcs are.

If you think about it from the humans perspective a bunch of aliens just came in and killed your entire family, then you finally get done grieving and all of the sudden here are the aliens again and they just killed everyone you've ever known. Finally through much grief counseling you start a new family and are on heavy meds to keep you from having horrific nightmares of your friends heads on pikes when out of nowhere zombies come and eat your unborn child right out of your wife.

Now those zombies and those aliens are playing on the same team? How could you ever put aside a hatred that intense to look at the world from their point of view when the two people that just murdered everyone you've ever know now appear to be friends and also trying to kill you for the fourth time? Maybe some people can put aside the past to work towards the future, but I know the vast majority of the population will just want blood for the innocent blood that has been spilled; regardless of whatever kind of backstory those aliens and zombies have.