Board Thread:General Discussion/@comment-20311922-20130916143753/@comment-11051098-20130918155603

Maybe he demonstrated his Shikai and Bankai, and also Senkei, to his grandfather Ginrei, making him the first person to ever see it?

Byakuya was meant to take the place as head of the Kuchiki clan and captain of the 6th Division, so I would assume that he demonstrated his powers to his grandfather to show himself worthy of following in his footsteps.