What causes very long, tortious colon with slow colonic transit, per sitzmarks (over half left in 5 days)? Does psyllium fiber cause redundant colon?

Yes. Fiber is healthy for general health and is helpful with colon function (i.e. Constipation). However, as with all good things there is another aspect. One side effect of a high fiber diet is enlargement and elongation of the colon. This can result in side effects like colon malrotation or torsion. Please know a low fiber diet leads to diverticula formation and the increased risk of diverticulitis.
Usually this is. Caused by chronic constipation and long term laxative abuse. Sometimes it is just genetic. Unless the bowel flips over on itself and obstructs or if dietary/ fluid management fail to resolve the constipation, surgery is not indicated.