What natural causes of an irregular period besides getting pregnant are there?

Reasons to be late: Reasons for missed/late periods: low body weight, obesity, marked weight ^ or v, over-exercise, endurance athletics, breast-feeding, ^ stress, illness, eating disorders (bulimia or anorexia), anovulatory cycle (no egg released), travel, hormone irregularities, drug use, meds (i.e., birth control) or medical problems (polycystic ovary syndrome or asherman’s syndrome). #1 reason is pregnancy.

Several: Hormone imbalances / variances; stressors; infections; trauma; low weight / poor diet; tobacco; stimulants; extensive exercise; low body fat; etc. There are many.