Are you sure this was sports motivated? My mother skipped a year and was bullied and isolated to the point of crippling anxiety because she was less mature and smaller than the other students. Obviously the school needed to deal with the bullying issue, but she definitely could've benefitted from spending more time with kids at her level of maturity. Arguably she should've stayed in the same grade but joined an after-school group with kids her own age, but sending her back a year and giving her enrichment classes would've also been a legitimate decision to consider.

It seems like this kid was in the right grade for his age, unlike my mom, but he still could've been immature for his age group.

I really hope their decision wasn't purely motivated by sports, but looking at the other comments, this seems like a depressingly common practice. Holding a kid back or skipping a grade is a serious decision that should only be done when necessary.