Subjects of Equestria! We are pleased to kick off the Nightmare Night Festivities and the 14th anniversary of the return of our beloved Princess Luna! Join us in celebration!
Notice how the front wheel appears to be slightly lower than the rear wheel. From other screencaps we can see all the wheels have the same diameter. Together with the slight downward curve of the road in the background behind the cart makes it seem like the cart is actually going downhill in this scene.