My guess(es): It may not really be shutting down completely. Do you stay around to see that the screen goes black and the power light ( assuming that 2ft iMac has one ) goes out? Does the drive go silent?
Or, the Mac thinks it was shut down because of a power failure and you have "Restart after power failure" set to "Yes". Or, more likely, the byte that holds the value for that last preference is set incorrectly to 1.
Have you checked the Apple Forums/KnowledgeBase?