The other day - as had happened a few times before - when I plugged my iPhone/iPad into my Mac iTunes failed to automatically start. Indeed iTunes didn't even seem recognise that my iPhone was plugged in - neither when directly plugged into USB nor when trying to 'sync over WiFi'. Another odd symptom was that my iPhone suddenly started to ask if I wanted to Trust this computer (that I have always synced to the past). A reboot had usually fixed this in the past but that's a pain so I snooped around for other answers - The usbmuxd daemon seemed to be complaining about weird stuff in the logs so I figured lets try and kill it! And low and behold that fixed the problem - usbmuxd restarted (probably care of launchd) and all was well again - iTunes saw my devices and wanted to talk to them again - a handy quick fix.
Here's the command line way to kill it - to be safe quit iTunes and unplug your iPhone beforehand: