Ah, the first female to touch this thread.
I have actually made this post here about a year ago. I was wondering because it does the same thing to me on occasion.
Nicotine and tobacco is something that stimulates bowel movements. (Hence why some people can't go without a cig)
It has to do with the tobacco, the air you are breathing in (with the constant deep hits, your innards are expanding and contracting which similulates pushing...) and the relaxed state you are in.
ah. I found it