Open Manage | Database | relationships.
Look for any relationships that link your layout's table to the portal's.
Double click them and see if the "delete" option was selected for the layout's table such that deleting a record in the portal's table automatically deletes any related records in the layout's table.
You may also be interested in the Delete Portal Row script found in the Known Bugs List database. It's a fully portable script--imports error free into any FileMaker database and the same script can be used with a "delete" button in the portal row of every portal in your file. (All layout and database specific information is passed as a script parameter to the script.)