When doing a shutdown with -F option to force fsck, on
reboot fsck is called with option -f, which is (if I got it
right) unknown. fsck exits with an error, and the initscript
prompts for the root password to go into maintenance mode.
I'm not sure this is the explantion, though.
This should be fixed in the next initscripts release
(which will be 3.91-1)