There’s a great story about this. It’s probably apocryphal, but it’s a fun enough story anyway. Like any such myth, there are variations, but this is the one I first heard.
There’s a tradition in classical music of standing when the Hallelujah chorus of Handel’s Messiah is performed. The story goes that the reason for this tradition is that at its premiere, King George II was so enthralled by it that he rose from his seat. And respect dictated that if the king was standing, so must everyone else be, so the entire audience stood.
Hegar