Ok, here is my views. Take it or leave it.
Britney Spears basically died (in the eyes of her fans) after Oops got off the top 20. She tried several attempts to come back (Toxic, ect).
However, here is my personal opinion as to why she is so insane.
The Papparazi.
The news media is at her home/work/ect 24/7. She has zero peace. They refuse to leave her alone. If your privacy was invaded constantly, I would understand her going a little nuts. The media has especially ripped on her mothering skills. I have zero doubt that Mrs. Spears would make a great mother, if only the reporters would leave her alone for a few days.
Funny I say that junk, seeing as I'm planning to go into Broadcast Journalism. :O