I think it's safe to say, Shanks got no other choice, but to make the first move, to intercept Blackbeard, he has to, because now that Kaido and big Mom isn't around anymore, supernovas are taking charge, He knew that Blackbeard will come after them ALL, especially Luffy, now that this chapter even further confirms that Shanks is the type that protects friends at ALL COSTS, he couldn't afford to let Blackbeard running around and start butchering all the people he cared about. Shanks is definitely a peaceful type, but he knows whats coming, the strong is starts to slaughtering each other, the situation demanded him to be aggressive. He has no other choice right now.