yes, it actually is because you are "massaging" your bowel. Running makes it bump, cardio makes you move your whole body, etc... if you could run without moving your upper body, you wouldn't shit as much
doesn't have that much to do with hydration. you wont seat a lot making abs, but you'll still shit a LOT!
do 100 abs every hour or 2, you'll shit your body out
BTW, best way to build-up your abdominal region is actually to lift your legs, not your body. leave your upper body in a fixed position, for example when you do a chin-up, leave your arms up, and lift both your legs at the same time. You'll have your 8 pack done right.
Also, push-ups are quite good at it too. it doesn't build-up arms as much as people think.