This is a serious high-level admin discussion going on here.

See, there’s one problem there: I’ve seen horses rolling on the ground, and it doesn’t look like that.
 
I think what we are actually seeing here is what I will henceforth refer to as a Pinkie Shuffle (it was either that, or upside-down worm shuffle).

Horses often roll around in the ground to scratch their backs - it’s a natural thing for them to do. (Of course, it’s also tons of fun if the horse you happen to be riding at the time decides it really could use a back scratch…). As for the stretch and leaping into the distance, that’s all Pinkie.

I did try and figure out what in Equestria it is she’s doing, but then I remembered that it’s Pinkie. Expecting things she does to make sense was rather silly of me.