After watching the video and/or seeing Tux perform his trick in person, many have suggested he has worms (gross, I know - sorry). Thankfully this doesn't seem to be the case, and because Tux shows off his trick only after spending time outside, our best guess at this point is that the grass and leaves in which he plays make his rear end itch.
Whatever the case, Tux's trick is adorable. I especially love that in the video you can hear me ask him to do it…and then he does. What an obedient little buddy.