Probably Thiago will explain this more professionally this is copied from Wikipedia:
In the vault final, Carey finished eighth. In the running phase of her first vault, Carey was off on her steps and performed only a tucked Yurchenko, a vault with a difficulty score of 3.3. She went on and successfully performed her second vault, an Amanar with a difficulty of 5.8, but was deducted two point points for performing two vaults with the same entry and for performing two backwards rotating vaults.
The expert on Eurosport said that a less experienced vaulter might have crashed into or under the vaulting table instead (if I remember correctly)