Madonna never had a BBL, she has implants.
A BBL takes fat from the stomach area and inserts it into the butt, but Madonna has always been too thin to have enough fat to graft for such a procedure.
If she did get her implants removed, I'm guessing she may have ruptured one of her silicone butt implants during rehearsals (maybe she fell on her butt?) and then they started leaking into her body which caused the bacterial infection/sepsis hours or days later.