Falling from a great height and shitting yourself midair.
Instead of a regular shit
on solid ground, which would naturally fall toward the earth, a dark rising occurs when in free-fall. Due to the sheer terror the experience of falling from high above might produce in a person, they shit themselves whilst they descend. The person falling and the shit loosed from their asshole is situated such that the shit evacuates and begins falling with a slight delay, thus giving the appearance of it "rising" out of their ass.