Why does Coffee + Cigarettes= POOP?

I have a "friend" who just drank a cup of coffee and then smoked a cigarette. Before he could finish the cigarette, an ominous feeling overwhelmed his bowels and he rushed madly to the bathroom. The carnage that ensued was legendary. Why does my "friend" have to poop whenever he drinks coffee and smokes cigarettes? Is it science? Sorcery?