Do you mean the window is really minimized (as in open, on the taskbar, still showing the folder, but minimised via the __ button), or that no windows are open?
Could you have a look at the change events Opus is receiving to see if there's anything unusual there? The Debugging section near the bottom of this post has instructions: Changes to folders are not being detected
Making a few process snapshots while the CPU usage is happening and sending them to us may also reveal something: Manually generating process snapshots