Her bum, when impacting with an object, causes it to vibrate at it's resonant frequency, sustained contact or large impact causes it to shatter. Or maybe she just hits it really hardIt's like she's dancing to some background music, unaware her flank is impacting with objects causing them to break. By the end of the song half of Ponyville will be destroyedNice you painted happy Derpy, seems all the art I see of her nowadays is her looking sad, good to see her smile againAll hair your artwork and the mare with the iron plot