Normal memories are stored by a part of the brain called the hippocampus.
You can think of the hippocampus as a sort of librarian which catalogues
(processes) events and stores them in the right place. However, some traumatic
events (such as accidents, abuse, disasters, or violence) are so overwhelming
that the hippocampus doesn’t do its job properly. When this happens memories
are stored in their raw, unprocessed, form. These trauma memories are easily
triggered, leading them to replay and cause distress over and again.