It's called vocal acrobatics. Christina Aguilera does it through 90% of her songs too. I find it highly annoying. All of the "hip" kids are doing it. I was forced to watch American Idol a couple of times and all of the contestants did it too. It's like trying to listen to a 4 minute, unstructured, out of control guitar solo. It gets old very quickly.
When singers do it while singing the National Anthem, I feel the need to punch them in the neck.
(Side note: It's a shame that Christina dresses like a 3-dollar potato, because she has a tremendous voice. Mariah used to showcase her voice too, now she just tries to jump through hoops.)