Not enough fiber! Diverticulosis is an incredibly common condition of the colon that occurs over many years. The colon moves waste through by "sensing" that it is there so bulky stool makes the colon more efficient. When our waste is not bulky due to inadequate fiber and fluid intake the colon has to work harder creating more pressure inside the colon like a balloon and the pockets (tics) pop out.
Diverticulosis. Diverticulosis is essentially caused by pressure. Straining at stool causes pressure within the colon, and that pressure seeks to escape through weak points in the intestinal wall. When escaping, the pressure takes some of the intestinal lining along for the ride causing pouches (diverticuli). Low fiber diet leads to straining at stool, hence the recommendation of high fiber diet.
Low fiber diet. Genetics likely plays some role. However, low fiber diet and other things that hinder normal bowel function are the most significant factors. Don't strain on the toilet!