how and why did mercutio provoked tybalt?

Certainly Mercutio had a sense of humour that got underneath Tybalt's skin. Tybalt didn't laugh so much and Mercutio's taunts enraged him. Tybalt asks Mercutio if he "consorts" with Romeo. Those were fighting words. Mercutio's relationship with Romeo was complex. Tybalt's threats ensured Mercutio would push him over the edge.

Tybalt was already all hot and bothered over (as he thought) Romeo's crashing of the Capulet party. When Tybalt shows up to confront Romeo, his threats drive Mercutio insane. As I mentioned before, Mercutio's feelings for Romeo are complex. It doesn't take long for Mercutio to taunt Tybalt enough as to have Tybalt turn on him instead of his friend Romeo.

can you explain

Mercutio's feelings for Romeo are complex. It doesn't take long for Mercutio to taunt Tybalt enough as to have Tybalt turn on him instead of his friend Romeo.

reword?

Mercutio forces the fight on Tybalt. Tybalt is really only interested in Romeo, but Romeo is not disposed to quarrel with his wife's cousin. Mercutio takes up the quarrel on Romeo's behalf for no reason other than that he is quarrelsome.

Mercutio and Tybalt fought because Tybalt had challenged Romeo to a fight but Mercutio knew that Romeo was not strong enough to fight Tybalt so Mercutio took the burden of fighting Tybalt and eventually dies when he is stabbed by Tybalt under Romeo's arm. Romeo had ran out to block the fight and Mercutio got stabbed. Tybalt leaves, and then returns clearly angry. He is then slain by Romeo and the Prince, Montague, and Capulet and Lady Capulet come out. Romeo exits, and Romeo is banished.

By complex I mean that Mercutio was actually in love with Romeo. Some might chose to debate this but there is enough textual evidence to warrant this conclusion. I have seen many productions of R and J where Mercutio's love and sexual frustration for Romeo is played up. I won't enter into that debate now but my take is that Mercutio taunts Tybalt because it is all that he can give to Romeo that Romeo will accept. Tybalt is by nature a comic sort of guy but observe how quickly his humour turns to angst when Romeo is involved.

Why does Capulet interfere in all of this?