He challenged Romeo to a duel.
Tybalt sent a letter to Lord Montague to challenge Romeo to a duel. Tybalt was angry that Romeo had attended the Capulet party and wanted to confront him for his presence.
Tybalt sent the challenge because he was angry that Romeo came to the party and that Capulet scolded Tybalt for not enduring Romeo. Tybalt was embarrassed by this and wanted revenge
Tybalt sends a letter round to the Montagues' house to challenge Romeo.
Romeo knew that Tybalt wanted to fight him because Tybalt had sent a challenge letter to Romeo's friend Mercutio. Mercutio then informed Romeo that Tybalt was looking for him to seek revenge for crashing the Capulet party.
Juliet does not know about all of those things we just saw happen in Act 3 Scene 1: the fight between Tybalt and Mercutio, the fight between Tybalt and Romeo, Tybalt's death, and Romeo's banishment.
Tybalt wanted to duel Romeo :P
Tybalt sent a letter to the Montague household to challenge Romeo to a duel. Tybalt is seeking revenge for Romeo's presence at the Capulet party and wants to defend his family's honor.
romeo was sent to mantua after killing Tybalt
Tybalt sends Romeo a letter containing a challenge.
Nobody sent Tybalt. Tybalt sent Tybalt. Why? Because he's decided he doesn't like Romeo and Capulet stopped him from fighting at the Capulet party in Act 1 Scene 5. Tybalt has decided to take matters into his own hands by issuing the challenge.
Challenging Tybalt was a rash decision by Romeo as it escalated the conflict between their families. Romeo's actions led to tragic consequences, contributing to the downfall of himself and Juliet. It would have been better for Romeo to find a peaceful resolution rather than engaging in a duel.
Tybalt has vowed revenge upon Romeo and has sent a challenge to Romeo's father's house.