The interesting thing is that she did stop drinking and still died.
Could be there was something else wrong with her. Maybe she just overcame an illness. Maybe her enzymes or lipids or proteins or something were out of whack. She could have had low potassium, high blood pressure, history of migraines, an infection.
There's a thousand reasons that the water could have triggered something rather than caused something. Anything out of place or not normal on her last medical exam will be brought up as a possibility.
That's how it always happens. It takes time for the huge amount of water that's entered the digestive tract to migrate around the body and into the cells which will be destroyed by it. At first the body tries to use its natural mechanisms to deal with the overload, and that delays the effects for a while, but with this amount of water it will often be unable to keep up. The exact amount it will take to kill someone is not easy to determine in advance, because various factors affect the equation, the chief one being the state of hydration and electrolyte levels at the beginning. Popping a few salt tablets at the beginning of the contest could make a huge difference in what the lethal dose would be, but there's still a limit.
Hopefully this incident will get enough publicity that FINALLY everybody who doesn't keep their head firmly buried in sand will wake up to this basic fact. There have already been at least 2 cases of children being killed by forced water drinking "therapy" recommended by quack behavioral therapists (with the Utah one being fairly recent and very widely publicized), a recent fraternity hazing case where a participant died from excessive water drinking, and several cases of marathon runners dying from drinking too much water (and that was without anyone pressuring them to keep drinking when it was making them uncomfortable, just overzealous following of advice "make sure and drink enough water to avoid dehydration").