The first thing that sprang to my mind was the TV series "the Avengers" back in the '60s.
It could be the movie's title would infringe on a copyright of the TV series title. Or.....the studio doesn't want people to think they're going to see a movie about John Steed and Emma Peel fighting space invaders!
Oh...bizarre. Yes, maybe to avoid confusion with the TV show 'The Avengers', though given that was made in the 1960s, I'm thinking their core demographic for this movie will be just a tad different..... Alas funny englishmen with bowler hats and sword sticks probably don't feature in this film ;) Curious decision all the same!