Why do you think Tybalt approaches Meructio and Benvolio and has a word with them in Act 3, Scene 1 of Romeo and Juliet?

Tybalt approaches the Montagues because he is angry at them for going to the Capulet party.

There is a feud between the Montagues and Capulets that has been going on for quite some time.  They are willing to kill each other on sight for the smallest insult, or just because.  When Romeo goes to Juliet’s party with his Montague friends, Tybalt Capulet recognizes them.  He is very angry, and tells Lord Capulet so.  Capulet scolds him and warns him not to cause trouble at the party, but Tybalt will not let it go.

Patience perforce with wilful choler meeting
Makes my flesh tremble in their different greeting.
I will withdraw: but this intrusion shall
Now seeming sweet convert to bitter gall. (Act 1, Scene 5)

Lord Capulet tells Tybalt that Romeo has a good reputation for a Montague.  Tybalt doesn’t care.  He is annoyed with Romeo for being at the party, and wants revenge.  Tybalt apparently has a reputation too.  Mercutio believes that Tybalt has a temper.  He calls him “prince of cats.”  When Tybalt shows up and says he wants a “word” with them, Mercutio counters that it will likely be “a word and a blow.”

Tybalt wants to fight Romeo for having been at the party, but Mercutio intervenes.  Of course, this turns bad quickly.  Mercutio knows that Romeo is not inclined to fight and will try to talk to Tybalt.  He sees him as family and not an enemy, since Juliet is his wife.  Mercutio knows that it won’t work, because Tybalt is spoiling for a fight.  He doesn't know about the wedding, but he knows Romeo is a Montague.

TYBALT

Romeo, the hate I bear thee can afford
No better term than this,--thou art a villain.

ROMEO

Tybalt, the reason that I have to love thee
Doth much excuse the appertaining rage
To such a greeting: villain am I none;
Therefore farewell; I see thou know'st me not. (Act 3, Scene 1)

This fight ends in Mercutio's death.  Tybalt kills him trying to fight Romeo, and Romeo tries to intervene.  Then Romeo kills Tybalt.  It's a big mess just for who went to a party, but that's the point.  Feuds are silly and harmful and people end up dead for no reason.  This unfortunately is not the last death, as we know.