User blog comment:Xilinoc/Ch. 569 (You've been hit by / You've been struck by / Senbonzakura~)/@comment-8669536-20140219184338/@comment-1604252-20140221010203

Well maybe they just helped Rukia finally master it rather than just giving her bankai. I always wondered if she was training to master bankai, since they say that after you achieve it, you're supposed to spend at least 10 years training to master it before ever using it for the first time. So maybe Rukia was training to master it and the Royal Guard helped her.