Answer: Excessive movements of the walls of the  abdomen
Basically babies belly breathe, that is they undergo abdominal pattern of respiration( instead of the abdomino- thoracic pattern in  adult), through their abdomen. 
<u>This is because,  their thoracic walls are not fully functional to undergo  the normal adult mechanisms  of breathing</u>. 
<u>The ribs are short in length due to incomplete ossification. They curve out wards, at low angle, their sternum, is shaped horizontally.</u>
 These features reduce the activities   of the inter coastal muscles and the thoracic cage  in the mechanism of  infants breathing,compare to the long ribs, obliquely shaped rib cage,  with lower  sternum  of  adult which  permit the  abdominal-thoracic  breathing pattern.
Therefore ,when the diaphragm distends during breathing ,its movement pushes the organs of the abdomen down; because infants have rapid respiratory  rates, the distends created  rapid  expansion and contraction of the walls of the abdomen to gain enough abdominal pressure. 
This together with the movement of  organs of the abdomen   from  the  pathways of diaphragm  created the  excessive  movements of the abdominal walls. This rapid abdominal wall movements creates discomfort and distress for the infant during this mechanisms.
Explanation: