Why does the lock on the passenger side door (front) keep locking by itself?

This could be any number of problems. It could be a bad door lock actuator, an adjustment problem with the latch and striker, body control module problem, wiring issue, etc. I would recommend having it properly diagnosed before replacing anything. If you would like to have us diagnose the problem please call us for an appointment. My name is Scott from Schaumburg Automedics. We are located at 503 Lunt Ave. Schaumburg 60193 (847) 352-2886. Please call me with any questions. Thank You