I had been plagued by an issue on my Macbook Air the last couple of months where iTunes would relaunch mysteriously. Not random relaunch mysteriously, more like, as soon as I quit, it would relaunch. I tried force quit and restarts. I trashed cache files and rebuilt the library to no avail. I filed bug reports and feedback to iTunes. Finally I reached out to @AppleSupport on Twitter. Right after (mind you, I DM’ed the account as they asked and so far no further reply) my internet friend @shep linked to a blog post of his from 4 years ago with his own saga of iTunes relaunching. I read through, and while I didn’t have headphones plugged in, I have dozed off a few times with my headphones in, and maybe, as he read about the microswitch in the jack, maybe, just maybe the headphones pulled out slightly & not depressing said microswitch. So I forcefully inserted some headphones and quickly pulled the plug out of the jack. I quit iTunes and so far, iTunes hasn’t relaunched.
Did I mention @AppleSupport hasn’t followed up yet?