Why are my users are being forced to reset passwords on almost every login?
Out of the box, Drupal does not have a way to force reset password automatically for users, I am guessing there must of use of any contributed module or there might be custom module written.

One of the contributed module which could do this is Force Password Change, can yu check if you have this module enabled? If so disabling the module or changing the configuration could help, also can you list down all the enabled non-core modules you have, you can get the list by running the following drush command, that will help in identifying if there is any such module used.

drush pm-list --type=Module --no-core   --status=enabled