They saw how Sunset can run fast that leads CHS to a massive victory againt Crystal Prep in soccer, it was technically her horse speed since she was from Equestria. So basically, she's still this fast, even when she's a human in her current home.

So, Applejack challenged Sunset to see if she's faster than any of the horses at her stable.

_{But we can't forget who always been the fastest person in the group.}