in my experience, you have to log in to your avast account in order to be logged in to the avast forum, so you must have had the password for your avast account, and logged in to your avast account, in order to be logged in to the avast forum.
i have never heard of a situation as described in the email, where a password-leak was detected by a third party and so one's account-password was reset.
i have heard of databases being hacked, where passwords were reset. like, for example, if avast got hacked, then they might reset everyone's passwords.