What are the causes of occasional green stool/bowel? I didn't eat anything green/vegetables.

See below. You may wish to visit this site for info on this topic:
Have a healthy diet, exercise, drink plenty of water, have safe sex, and get HPV vaccine. Wish you good health!
Nutrition: http://www.choosemyplate.gov/.
Depends. Sometimes is the bile juice, a normal secretion of your digestive tube, that gives this color to your stool. If you have no symptoms whatsoevere probably this is the reason.