Generate new password for users periodically and notify user of new password
This issue was talked about during the great Drupalgeddon. Because no such bulk reset tool existed for a site.

Here is a recently created tool to perform this task. Other such tools have existed since Drupalgeddon.

Mass Password Reset Module


  This module allows users with "Administer users" permission to reset
  all user accounts and notify all users
  
  Details
  
  The module will reset the passwords of all users except the
  administrative superuser id 1. You can optionally reset the
  administrative superuser id 1. Once passwords have been reset, users
  will optionally receive emails using the Drupal password recovery
  email. The password recovery email content can be edited at the
  account settings configuration page.
  
  Use Cases
  
  You have a large number of user accounts pre-created and
  want to send password recovery emails to all users during a site
  launch. You need to quickly change all passwords on a site for
  security reasons.