How did Scarlett and Rhett's daughter die in "Gone With the Wind"?

Eugenie Victoria "Bonnie" Butler (1869-1874) was the daughter of Scarlett O'Hara and Rhett Butler. Bonnie was much like Scarlett: beautiful, vivacious, charming, and spoiled.  Bonnie was described as physically resembling Scarlett as well, but she had inherited her grandfather Gerald's Irish blue eyes.

Bonnie was much loved during her short lifetime, particularly by her father. As the relationship between her parents crumbled, Rhett devoted more and more of his time and attention to his daughter, to such an extent that Scarlett felt that she was being cut out entirely. Bonnie was born in 1869 to Scarlett and Rhett in Atlanta. She was initially given the name Eugenie Victoria, but only hours after her birth she was given the nickname "Bonnie Blue", after Melanie remarked that her eyes were as blue as the 'bonnie blue flag' of the Confederacy. The name stuck, wiping all memory of Eugenie Victoria.

Rhett who for once is stern, says Bonnie can't have a bigger pony until she's six. Bonnie pouts and begs Rhett, who melts and decides that Bonnie can jump higher but not to blame him if she falls and gets hurt. A few days later Bonnie excitedly starts to jump over the fence and proudly shows Scarlett. Scarlett begins to watch Bonnie proudly but her pride soon turns to fears when Bonnie jumps over and extremely high fence.  Scarlett yells for Bonnie to stop but sadly Bonnie falls down on the ground and breaks her neck.