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.)