Okay, let’s give some context to this image: Luna having recently been reformed from Nightmare Moon in the season 1 intro, had noticeably been aged backwards to her adolescent years. Given that alicorns are technically immortal (we’ve never seen one die before) this came as a minor inconvenience at best, considering that all the lost time in the last one thousand lonely winters could now be completely made up for, she needed a second chance, and not just Luna but for Celestia to finally give her the love and appreciation Luna so desperately needed in her formative years. Unfortunately, at the time Luna was not as mentally developed as she needed be to exercise authority in the Kingdom, and when Twilight came as a stuart to exercize authority for Prince Blueblood, who notably did absolutely nothing but take advantage of his social status and mooch off his auntie, Luna mistakenly assumed she was being replaced, and this simple misunderstanding spiraled into a tantrum of godlike proportions (alicorns are such entitled brats). Twilight, being knee deep in stress and anxiety gave into her sadistic anger and proceeded to punish Luna herself, and Celestia hearing the comotion down the hallway entered the room shocked by this unprecedented and highly inappropriate display. She gave Twilight a firm lecture on how she was supposed to do it on the bare bottom and then proceeded to join the spanking.