What causes diverticulosis?

If you mean : Diverticulosis coli, no one knows for sure. But the prevailing theory is hypertonicity of muscles of the colon. This pressure squeezes down on the mucosa so hard that it pouches out at weak spots where blood vessels penetrate the wall of the colon. Diet that is low in fiber causes low volume stool, which in turn causes the muscles to squeeze harder than it should. Other organs are different.

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.