I'm on a MBP running OS 10.6.8, not Windows.
This is a noted problem that has come up before. An update to BS2, not sure which one, added an auto save that happens at a regular interval. Before that it only happened while you were idle. Dr. Smith has said there would be an option to change the behavior but it hasn't come out yet.
Since BS was developed for Windows for awhile now and is just coming to the Mac, I suspect this may only be a Mac problem.