Hitting stop makes the session "changed"

Why does hitting stop make the session indicate it’s been changed with an asterisk, even though I have not changed anything? Literally, hitting stop when playing creates this condition. I’ve actually noticed this for a long time in Ardour, but have decided I’d really like to know why this happens. It does seem to happen after I’ve done a lot with a session, so I’m sure it’s something I did in the session that makes this happen. Fader automation? Something else?