Recent versions of MacOS reopen windows for you because it's convenient or something. I think it's a really confusing user experience, and rarely what I want the system to do. I'm primarily a Mac user at this point, and it totally freaks me out on a daily basis.
Anyway, do the following:
Open Finder and eject the Adobe Flash Player disk image from the left-hand column, if it's still mounted.
Then log out of the machine, but uncheck the "Reopen windows when logging back in" on the logout dialog.
