Shanks saved Sengoku, Garp and Marineford island from BB. BB would sink the island otherwise.
Shanks simply knows how strong BB is and warns people to not fight him. He is trying to prevent people to fight BB, LOL.
Its not fated anything, this is nothing more than ZKK, Shanks might fight Luffy or someone else and Bb can steal poneglyh without fighting him.