It is a (very good) theory that the mother was in a rage about a bed wetting accident and was already pissed that jonbenet was refusing to wear the same outfit as her to a christmas day party. Many experts say toilet matters are very often a trigger for rage.
JonBenet was in the bathroom and the mother ripped the overnight pull-up off the girl (leaving a mark in the vaginal area consistant with that action) and when the girl fell back she hit her head on the bathtub. This is consistant with JonBenet's injuries. The mother believes that the unconscious girl is already dead and has to make a decision - call police or cover up. She decides to make it look like an intruder did it.
She brings the girl to the basement, bounds her, writes a ransom note, calls 911. All consistent with evidence.