"Logon failed for login 'sa' due to trigger execution." When No SA Login Triggers Defined
"Logon failed for login 'sa' due to trigger execution."


For security reasons logon triggers don't report errors to the end user, but they are captured in the SQL Log.  From Docs:


  all messages originating inside the trigger that would typically reach
  the user, such as error messages and messages from the PRINT
  statement, are diverted to the SQL Server error log. 


Logon Triggers


  Is there another explanation for a logon trigger not allowing a login that it does not check against?


Of course you could always have a bug in one of your logon triggers.  Check the SQL Logs.  And of the Logs don't clearly indicate which trigger failed and why, then add more telemetry (PRINT or RAISERROR) to your triggers so that next time they will.