What is causing iTunes to open?

This is a super old question, but I can admit to giving up looking for a source for a rather long time. It turns out (in my case) that the magic "4th conductor" within the headphone port that is used for the microphone also is used for the remote control. ie: Vol up/down, Play/Pause, Next/Prev, etc. This works by shorting that 4th conductor in various ways. When the cord for my headphones, with only 3 conductors, wiggled in it's loose fit, my laptop thought that I had pressed the play button, which triggered iTunes to open up.

(I have also seen a similar thing happen with bluetooth headphones, as mentioned above.)

I was also experiencing this issue. I had turned off all the auto downloads in iTunes. Then, in System Preferences, I "delete"d iTunes Helper - which did not delete anything and may not have had any effect. I discovered later that it was when I turned on/connected Bluetooth headphones that iTunes opened without any notice in the background.

I haven't found a way around this yet, but at least I know what was causing it.

This was with iTunes 11.4 and OS X 10.9.5

Of course this may or may not be what is happening with your set up. I would be interested to know if other things also cause this.

I just had this problem start up. After an extremely frustrating troubleshooting process, I realized my Mac was accepting commands from my Logitech universal TV remote (which I was attempting to use with my TV). Every time I pressed the OK button on the remote, iTunes would start to play. I had never attempted to pair this remote with my Mac.