I think it's the styling methods used and her haircut. Before her big chop, she had a bad hair cut. She always had slightly frizzy, pyramid hair. Once her hair began to grow out, it looked as if she had a cut that worked with her curls and she used better products.
-- I don't think she has a relaxer because many of the pics of her when she is just about town show her with curly hair pulled back. I think she just has great stylists who are pros at making her hair look naturally straight.