Based on the information provided, Steve's memories are said to be experiencing retrograde amnesia.
Retrograde amnesia is a type of memory loss in which a person is unable to remember events that occurred before a particular point in time, such as an injury or trauma to the head. In Steve's case, falling off his porch and bumping his head could have caused a head injury that resulted in retrograde amnesia. This can cause memories leading up to the event to be more difficult to recall than memories that occurred after the event.