Why do you think Gregory and Sampson behave more aggressively as soon as they realize that Tybalt is approaching?

Gregory and Sampson behave aggressively in front of Tybalt to show loyalty and support to their master, Capulet, who is engaged in a feud with Tybalt's family, the Montagues. They want to assert their allegiance and make it clear that they are willing to defend Capulet's honor against any threats, including Tybalt. Their aggression also serves as a way to intimidate Tybalt and show that they are ready to confront him if necessary.