Bowser was minding his own business, when he found a dead Toad in his castle hallway. He decided to eat it, because why not.
He loved it. He wanted more. He went to the Mushroom Kingdom to find more prey. He killed many Toads, which is something he's never done before. He cooked the Toad corpses with his fire breath and ate them. His diet caused his scales to turn red and his pupils to dilate. After about a week, Bowser wanted to expand his diet. He killed some Yoshis and a Goomba, cooked them, and ate them. Later, he killed Luigi and Wario, and ate them.
Mario doesn't know about any of this, but he starts to suspect that Bowser kidnapped his friends, so he goes to Bowser's castle to stop him.
"You'll make a perfect buffet for the wedding, little plumber."
