He didn’t so much “fall” off the piano, but more or less “launched” himself off the piano, in some sort of physics-defying accident. First things first, he’s completely fine after the tumble, and continued on with the show like nothing happened. Check out the video of the jump/slip/fall/launch just below.
I love the joke he told jsut after the event about how he sits on the computer with his father and YouTube “Twenty One Pilots falling.” Glad he’s ok, and still has a great sense of humor about the situation. Very real guys!