Savers can crash without the screen unlocking. Are you sure it was xscreensaver you were running, and not one of the innumerable incompetent knockoffs?
Sounds like you were probably using gnome-screensaver or some of the many other poorly written alternatives like cinnamon that do this. I don't believe there is any way for xscreensaver to unlock the desktop even if it does crash
Incorrect. It’s a limitation of X11 that if the screensaver daemon crashes, including xscreensaver, the desktop will be unlocked. See the JWZ links that are posted in this thread.
