It was conjectured that the rudder trim may have been displaced by the jumpseat rider's foot or perhaps a meal tray set on the pedestal to serve coffee
Maybe so but I experienced a rudder trim runaway in the B737-300 simulator. If you tweaked the knob and let it go (as you would), the damned trim slowly ran to full deflection. During that session the instructor was constantly criticising my handling of engine failure after take off as I was having considerable difficulty in keeping wheel central and applying correct amount of rudder. Later we discovered that each time I touched the rudder trim after it had been centralised at my request by the PNF the aircraft started to rock and roll. This was due to the rudder trim going uncommanded full scale.
Had the same experience in a real 737-300 while doing trim checks prior to start. Never trusted it after that and always double checked setting after making rudder trim changes.