Sabo got the mission to eliminate or capture strong pirates. As he saw Ace, he didn't hesitate to attack him. Since he lost his memories

Ace: Sabo? Sabo it's me! Ace!

Sabo: Is that supposed to mean something to me?!

Sabo manages to push Ace on the ground. But Ace flips over and shoots a fire blast on the ground next to Sabo's Face

Ace: Sabo calm down! It's me! Your brother!

Sabo fights back

Sabo: I Don't have any brother's! I would know it!

Ace: You have Me and Luffy! Your Brothers!