This theory came up in my mind while reading Chapter 1014.
First, we know that Shanks has a lot of info of the World. He probably knows the truths about the Void Century, the meaning of D., and the Ancient Weapons, although he has not been at Laugh Tale. He knew Kaido wanted to go to Marineford and he knew Ace was hunting Blackbeard and asked Whitebeard to stop him from doing that.
Second, I believe Shanks and Kaido did not fight during Marineford Arc because the Beasts Pirates are not easy to fight without going all-out and getting at least a few bruises, but Shanks’s crew was fresh when they appeared.
Third, Kaido’s words in Chapter 1014. While Luffy was falling in the ocean he said that “even him was not Joy Boy” or something along the lines. I feel that Kaido meant he already thought he found Joy Boy once in the past.
We know that Ace went to Wano and destroyed his symbol. At first I thought Kaido wanted to go to Marineford because he wanted to die in an epic way and because Ace, the guy who destroyed his statue, was there so he had to go to make sure he would die.
But the latest chapter made me think that Kaido thought Ace was Joy Boy. He invaded Wano as predicted by Oden before his death. He was the second strongest Commader of the Whitebeard Pirates. Lastly, he was the son of Gol D. Roger.
Shanks understood it and went to stop Kaido by giving him proof that Ace was not Joy Boy. That’s how Shanks was able to stop Kaido from going to Marineford.
*Theory by Western_Bear
