Drupal users getting blocked automatically
Do you happen to have the Inactive User module installed? This module can be used to block users who are inactive for some time. Otherwise, are you sure that the users are not trying to log in with the wrong password (5 times), which would also block them. It could be possible that someone else is trying to log in with these user accounts and is getting them blocked (I suppose it would be up to you to decide how likely this is based on whether or not your website is fairly high or low profile).