Multiple masters when deploying images

We use DS 6.9. Sometimes we get problems when deploying to multiple PC's. We had this problem yesterday. I deployed an image to 20 PC's and ended up with between 8 and 10 masters depending on when the deployment console updated the display. It looked like some of the PC's were running more than one download at the same time. In fact some of the PC's were alternating between master and client. Naturally with all these masters and clients running the times were in the hours. I left them running and eventually after a few hours they all finished successfully. Does anyone have a clue as to what's happening? Normally this job would take 45mins, usually works fine and selects only one master.