In my experience, some snakes are more prone to going while you're handling them than others.
I used to have a Pastel that would go, at least a little, everytime he was taken out of the tub. I always had a towel in one hand anytime he was in the other.
Most of mine have never pooped or peed during a handling session.
Many of them will go right after being handled...seems like the exercise get's their system moving.