CiviCRM WordPress Member Sync plugin keeps a WordPress user in sync with a CiviCRM membership by granting either a role or capabilities to a WordPress user who has that membership.
We recently imported a different membership (not set to sync members) via .csv file to all of the contacts in the system (over 900 in total), but after the import WordPress accounts were created for all the CiviCRM members/contacts. We have triple checked all of the settings and this definitely should not have created the WP accounts for this membership. We also tested adding this membership manually with multiple contacts (before importing) and no WP account was created. This suggests there may be a bug while importing members via a large .csv file.
From this issue on the WordPress forums: