It could possibly be more than just legal issues, though that could definitely be a part of it. It may also be a similar case to the whole Orion: Dino Beatdown thing. --- I asked God for a bike, but I know God doesn't work that way. So I stole a bike and asked for forgiveness.
More than likely they changed the name to distance themselves from the name War Z, since that was the title at release that was known by many as one of the worst games ever that essentially scammed people so bad Steam had to pull their game.