To the best of my poor knowledge, if Standby rudder on light illuminates when descending below 700ft you could suspect it's RPR failure to transition to normal press causing automatic switching to sby rudder n illumination of the light.
At other time the most probable cause is either manual switching of flt control switch to sby rudder or...FFM trigger. There's no way of knowing which system (A or B) that cause the trigger... This is on the -400 that is fitted with RSEP...cheers