iPhone pauses or plays songs when the headphone plug moves
It happens because a quick jog of the plug causes a short interruption in line level.  It's the same with headphones -- Apple or otherwise -- that include a play/pause button (usually place on the cable itself, where it splits to go to each ear).  That button functions in exactly the same way, by momentarily interrupting line-level on the phone's cables.  iTunes starts playing because it assumes the interruption from plug movement (or a dirty plug) is a play/pause signal.  As a simple demonstration, start iTunes playing something and wiggle the phones jack, same conditions as make iTunes launch and play, only if it's already playing this will make it pause.