Shirley, the problem occurs no matter how task manager is started. I've tested cntl-alt-del, running from the start-run dialog, creating links, creating shortcuts on object dock, and using the stack docklet. No matter how task manager is started and no matter how it is closed the problem would occur on starting and stopping task manager.
I would add that harrison1701's observation that "reserve edge of screen" was linked to the problem turned out to be correct for me. I turned that off on all docks and the problem went away.