Actually Waluigis just some guy who showed up to play tennis one day and everyone just went along with it. In brawl in the family (the comic I'm referencing), however, he's an insane crazy person who ocasionally takes over the comic in various, occasionally disturbing, ways.
So it’s eiher wario has been acting like mario trying to hog he spot light.
Or wario Met waluigi in a seedy mushroom kingdom bar.
“I hate those mario brothers. NeewH!... but mostly Luigi.”
“Pah! Haha! I hate mario!”