To add a bit to WhyNots answer, the reason it comes and fades, and then recurs (repeat as needed) is that Peristaltic waves move in a one way direction down the digestive tract. The waves push the intestinal/colonic contents toward the posterior opening. These waves are not conscious, but occur at regular intervals day and night. Often the cycle occurs in the 10 to 20 minute time frame, but not always.
The urge to poop comes when fecal material enters the rectum and approaches the internal anal sphincter. The internal sphincter opens, and then the external opens. When we are potty trained, we can resist opening the external sphincter, and wait for the next urge to void the bowels. Babies can’t control the external yet, therefore more frequent and usually softer poop.
One of the dangers of putting off pooping in adults is that the colon is a very good organ for the reabsorbtion of water…so the longer one puts off pooping, the more water is removed from the fecal mass…the result is dry, hard and smaller feces; in fact they may become little hard spherical masses and in extreme cases must be removed manually.
Can you tell I used to teach Anatomy? Poop as soon as possible folks! Avoid constipation and possibly hemorrhoids!