Are you using the WP Discourse plugin? If so, did you have in installed when the users were first created. The way it is supposed to work is that when a user is first created with the WP Discourse plugin enabled, their email address is flagged as being unverified. If the users are registered through the default WordPress registration process, that flag is removed and they are able to login to Discourse without having to confirm their email. If you are using a custom registration process, then users will have to verify their email address.
It is possible to override that flag, but I"m curious as to why it’s being applied to existing users.