A321
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Distraction
Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Illness / Injury; Ground Event / Encounter Other / Unknown
Flight Crew Took Evasive Action
Human Factors
On arrival at gate [in] ZZZ I went through my shutdown flow and stated shutdown checklist complete. I then moved my seat slightly aft to begin stowing my items into the kitbag when I noticed what I thought was the jet bridge moving forward. It took a moment to realize that the aircraft was actually moving aft. With my seat further aft than normal; I had to reach the brakes with my toes which brought the aircraft to a stop more abruptly than normal. I made a PA; then restarted Number 1 and taxied back to the gate.Somehow; I neglected to set the parking brake. When I set the brake; I look for pressure and look for the parking brake sign- not sure how I forgot it. From now on; I will verbalize parking brake set before turning off the seatbelt sign.A male passenger in row 24 fell over and his iPad struck a lady in row 24 on the forehead. FAs (Flight Attendants) gave ice to the passenger and when she left the airplane she turned down further assistance saying that it was 'just a bump.'I assume that I missed setting the parking brake because of a distraction; although I cannot recall what distracted me.Speaking for myself; I will now say; 'Parking Brake Set' before turning off the seat belt sign.
[Report narrative contained no additional information.]
A321 flight crew reported failing to set the parking brake; resulting in the aircraft rolling several feet after parking.