What causes a baby to come early but not prematurely early, say 2 weeks early when you are 100% sure on dates menstrual dates?

Nature vs. unknown.: The law of statistics says the *average* gestation is 40.5 wks, so that means some babies will be born earlier, some later. While there are many causes for premature birth (premature rupture of membranes, amniotic infection, adverse reaction to medications come to mind), a good percentage of the time, no cause can be found. The baby starts the process of birth. He/she will come out when s/he wants