I think it is just load on the server, resulting in what they call 'buffering'....that is, waiting while it is sending you enough of a file to continue playing. I see it often, and if it bothers me, I just hit 'pause' and wait awhile....sometimes till it finishes downloading.

   Some sites have more bandwidth & 'signal strength' than others, and it may well depend on the time of day and the popularity of the item.