I really think it is an electrical charge type issue. Anytime it has happened to me, my device is just sitting here on my desk within hands reach. The cord is not taught, its very loose with plenty of slack. And when it happened it was only some slight movement nowhere near enough to pull the plug out of the socket.
Now again this has not happened while it is plugged into my home system just sitting on the main power amp. I think it may be something to do (in my case) with being very near to my laptop, laptop screen and a monitor screen as I use dual screens. Maybe a static charge of somekind just enough to disrupt play and start the device over.
Again, I am not concerned about it, its not my device..it plays perfectly fine. Before this post, I have never mentioned this in the forums.
Zune is AWESOME!!...since Nov 2006 Black30GB and Pink30GB (my wifes'!!!)