Well when one edits a portal record, it locks the parent record as well. However you state that it locks all the records in the parent table for other users? Are you certain? What is your relationship like and keyed on what? Are you opening a new window?
As mentioned, this is common if you have a relationship that links many records together and a portal displaying info from that relationship is present on the page you are editing.
For example, if you had a relationship that linked all records with today's date, and had a portal using this relationship, and the portal contained a modifiable field, then editing one record from today would lock out anyone from editing any record from today.