Why are my users are being forced to reset passwords on almost every login? After logging out of my site, after about 30 minutes, each user's password is reported to be invalid -- that is, they cannot login with their existing passwords and are forced to use the reset link.

This happens almost every time and happens absolutely every time after a certain period of time. 

This is an extremely frustrating problem to diagnose -- I'm not even sure where to start. Any suggestions?

I'm on Drupal 7.50 using a bootstrap 3 subtheme -- heartbeat controls some features.
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