Clones know no fear. (or are brave)
Which is why they don't poo in combat.
Pooing when dead has nothing to do with fear, it just happens.
I don't think a human corpse would run out of ATP fast enough to cause instant relaxation and weakening of the sphincters in the time span while the corpse isn't yet "reclaimed" by map's floor. Rigor mortis doesn't happen the second a creature was killed.
I didn't have much experience with Sarge's biology though, so feel free to correct in context of military clones.
Happy 3500th post, by the way.