In Romeo and Juliet, how does Lord Capulet's behavior change dramatically between Act 1, Scene 2 and Act 3, Scene 5? What are the causes for that...

Lord Capulet goes from being a jolly father to a tyrant.

When we first meet Capulet, he seems to be a great guy.  As far as he is concerned, everything is going well.  He finally gets to show off his daughter.  He has a big party for her, and is a very congenial host.  You would think Lord Capulet was the most generous and forgiving guy in the world.  He even isn’t bothered by Romeo’s unexpected appearance at the party.  However, soon he is complaining that if Juliet doesn't marry right away he will disown her.

When Capulet and Paris first discuss Juliet’s marriage, Capulet is not ready to have her married yet.  He tells Paris that she is too young to marry.

But saying o'er what I have said before:

My child is yet a stranger in the world;

She hath not seen the change of fourteen years,

Let two more summers wither in their pride,

Ere we may think her ripe to be a bride. (Act 1, Scene 2)

This is the same mindset he is in at Juliet’s party, when he seems like nothing more than doting father.  He wants everyone to have a good time, and doesn’t even care that Romeo is a Montague when Tybalt complains about his unexpected appearance at the party.

Content thee, gentle coz, let him alone;
He bears him like a portly gentleman;
And, to say truth, Verona brags of him
To be a virtuous and well-govern'd youth … (Act 1, Scene 5)

Capulet does not want to make trouble at the party and upset his guests.  He tells Tybalt to leave things alone, because he has heard good things about Romeo.  In contrast to Tybalt, Capulet seems even-tempered and kind.

What happens?  The next thing we know, Capulet is ready to marry off his only daughter, despite her objections, to a much older man.

Hang thee, young baggage! disobedient wretch!
I tell thee what: get thee to church o' Thursday,
Or never after look me in the face:
Speak not, reply not, do not answer me;
My fingers itch.  (Act 3, Scene 5)

While we might have been disposed to like Capulet before, and think he was a good father, we are no longer ready to nominate him for the Father of the Year award.  He threatens to beat Juliet, kill her, or disown her for not obeying him.  He has every bit as much temper as Tybalt in this scene.  Juliet is defying him, and he will have none of it.

So what happened?  The difference between these scenes is that Tybalt is dead.  Clearly, Capulet is grieving.  Tybalt must have meant a lot to him.  Maybe he realized that life is short.

Capulet’s insistence that Juliet marry, suddenly and against her will, is a direct line to Juliet and Romeo’s deaths.  Juliet cannot marry Paris.  She already married the love of her young life, Romeo.  Juliet was ready to marry, just not Paris.  Capulet refused to listen to her.  He went from thinking she was too young to marry to insisting that she marry the man of his choosing right away.