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.)