Pregnancy is a clear reason for enlarged hemorrhoids though, again, the rationale is confusing.
With time, the hemorrhoidal tissue loses its mooring and slides down into the anal canal. One physical fact that is known about enlarged hemorrhoids that could be topical to knowing why they form is the pressure is raised in the anal sphincter, the muscle that surrounds the anal canal and the hemorrhoids.

The anal sphincter is the muscle that permits us to govern our bowel movements.

It's not known if this raised pressure predates the development of enlarged hemorrhoids or is the result of the hemorrhoids.

There are 2 kinds of nerves in the anal canal, abdominal nerves ( above the dentate line ) and somatic nerves ( below the dentate line ). The somatic ( skin ) nerves are similar to the nerves of the skin and are capable of sensing agony. The abdominal nerves are the same as the nerves of the viscera and don't sense agony, only pressure. Thus , internal hemorrhoids, which are above the dentate line, typically are painless. As the anal cushion of an internal hemorrhoid continues to enlarge, it bulges into the anal canal.

It may even pull down a little of the liner of the rectum above, lose its standard anchoring, and stick out from the anus. The rectal lining which has been pulled down secretes mucus and wets the anus and the encircling skin. The prolapsing hemorrhoid sometimes returns into the anal canal or rectum on its own or can be pushed back within with a finger, but it prolapses again with the subsequent bowel movement. Locked up hemorrhoids can have their supply of blood shut off by the squeezing pressure of the anal sphincter, and the blood vessels and cushions can die, a condition called gangrene. Third-degree hemorrhoids : Hemorrhoids that prolapse but must be pushed back in by a finger. Fourth-degree hemorrhoids also include hemorrhoids that are thrombosed ( containing blood clots ) or that pull lots of the liner of the rectum thru the anus.In general, the indicators of external hemorrhoids are dissimilar than the indicators of internal hemorrhoids. External hemorrhoids can be felt as bulges at the anus, but they usually cause few of the symptoms that are common of internal hemorrhoids.

This is maybe, as they are low in the anal canal and have tiny effect on the function of the anus, especially the anal sphincter. The thrombosed hemorrhoid may heal with scarring and leave a tag of skin sticking out from the anus.