2005 Micra Urbis 1.2 3dr central locking -driver's door locks when I'm in i?

have 2005 Micra Urbis 1.2 3d with central locking - when I'm in the car, the driver's door has locked from the inside - i can only get out if someone opens door from outside (it opens without any problem!!) or else via passenger . It's has only happened about half a dozen times in past year and I haven't noted any pattern with it