Baz has properly identified what is happening. There is an “auto” backup feature within SL, and in a default install, performs these backups at various times. I don’t currently have the time to review this topic in its all of its glory, but do promise to revisit this thread in two or three days and explain it in more depth.
For now, to see the options associated with the feature, visit the Options drop down menu, then select Automatic Save. For additional information on the feature, do a key word search of “Automatic Saves” inside the online Help file. One of the few times the CADlink Help file does a respectable job.