the "why" is very likely to be a leak.. the central locking system is pneumatic (air)
my old car ( '96 polo) i had this very problem, which turned out to be the motor for the 'spike' that controls locking for the petrol cap leaking air..
i didnt even know the petrol flapped locked since when i bought it the car as the spike had been snapped off at some point!
i know for a fact the previous owners had also changed the pump...
when i got it looked at they had to replace the 'spike' thingy.. along with the pump since the leak had caused the pump to knacker itself.. and probaby didn't know this spike thingy was knackered.
get the leak sorted quick... because central locking pumps run at around 150 quid