I see this question asked a lot. I am a studying neurscientist and this is actually do to the fact in our parasympathetic nervous system acetylcoline is a neurotransmitter that controls the development of tears from the lacrimal gland but also is released down the same cranial nerves until it branches out to the nerves traveling to your colon and other glands to increase hostile or peristalic movement of your bowels. Excess acetylcoline can cause this to happen. Stressing the anal sphincter increases rate of lacrimation.
It can be due to constipation. Usually, major cause of this condition is lack of fiber in diet. Taking of foods rich in fiber, regular exerciser and drinking of lot of water can help you.