After Rainbow Dash flies another aerial race in her Equestria Games outfit (that’ll be a white tank top with skintight white shorts), she spins out on crossing the finish line and lands in a large mud puddle. When she recovers, she decides to just play around in the mud. Dashie doesn’t mind getting dirty!
 
Rarity is initially a bit put off that Rainbow got her outfit messy, but decides to let it slide and appreciate the fun Rainbow is having, since the outfit was meant to get sweaty from flying so hard in it anyway, and last race Fluttershy’s outfit was soaked with sweat while Rainbow’s was damp.
 
Original picture didn’t come with a sports outfit but I edited one on. If you prefer for them to be wearing something while covered in mud (and thus Rarity in Sisterhooves Social isn’t enough for you), I can do one for Rarity as well. (Might not be super motivated for an Applejack one though unless I do one for her and Rarity having a friendly mud fight)