Nicotine is a fucking stimulant! It stimulates peristalsis (muscle contractions) in the digestive tract. At the same time it increases heart rate as well as constricts blood vessels, so...yeah...that pretty much explains it.
I had a really, really cool bio teacher in middle school!! I remember almost everything he ever said!! He got kinda over-excited talking about the whole digestion process, from mastication to defecation.
I don't know where I read this, but something in those cigarettes can do that to you, relaxing your bowels or some shit like that.
I'm taking about marijuana tho.