Acetylcholine.

There's plenty of things that can trigger it being dumped into your system. It's a neurotransmitter, and works throughout the body like most of them do.

One of the things that triggers it is eating. You eat, it gets released. It gets released and fairly quickly hits the receptors that are open to it.

One of those receptors in the gut then increases bowel/intestinal motility.

Thus, you wake up, and your body starts pumping out fresh chemicals. You eat, and the chemicals related to that crank up. Then, poop happens.

There's a shit ton of things besides eating that can get the job done, but it's extremely common for a post prandial poop to pop.

Me? IBS has fucked my shit up for ages, and I can usually time when that first rumble will hit after I start eating. Fifteen to twenty minutes. It isn't always urgent as that's determined by a wide range of factors, butt it'll definitely hit in that time range if there's no stimulants involved (and I'm real sparing with caffeine). So you aren't alone at all :)