login problems and password auti-resetting

Hi,
I have a WordPress site where I have set up a number of accounts. The roles of the accounts vary: some are authors, others editors, and others are admins. Some users have logged in to their accounts while others haven’t. Last week 2 users who hadn’t logged in before tried and were unable to. I investigated and found that the passwords had changed. This happened on 2 other accounts which had previously been logged into. None of the account information had been changed on any of the accounts. I tested all accounts before handing them out to be sure login was successful. Five of the accounts were either editors or authors, and one was an admin. It seems that the passwords were changed without any intervention. Has anyone else had a similar problem where passwords seem to change on their own?
Thanks.