@itsbhanusharma was right. If discourse is the sso provider, all the users are registered through it. Now if you change the provider to wordpress, they need to login through wordpress but they don’t have a wordpress password. Hence they need to reset their password via wordpress to get a password.
One scenario that is possible, is that if the users were already registered to wordpress before the sso was in place, they already have a wp password and can use it.
Moreover, we do have a way to create a discourse user upon them signing up for a membership or subscription even without switching the sso provider. They’ll get an activation link in their email to activate their discourse account.