It's knick name is peter patter  it is a mother thing a kitten does this
while they are nursing to stimulant mom to produce milk.  in an older cat it
is a sign of affection or comfort.  My experience shows that it usually done
by a cat that has been taken away from mom really  young or when the cat was
not ready to be taken.
   It usually is done to the person that the cat feels extremely
comfortable with.  My guess is that your wife feeds the cats and cleans the
litter boxes.  I have two cats that do this to my pillows on my part of the
bed  rarely does this to my wife.  My one cat that is extremely attached to
me will do this when he can lay on my back  except he is not declawed  ouch