ELI5 - Why does that first coffee in the morning trigger a bowel movement?
When we sleep, everything in our body is slowed down and our digestion is pretty much on pause. (That's why you aren't supposed to eat heavy food before bed.) Your brain will suppress urges like going to the toilet. Once you wake up, your body slowly resumes all those functions. Coffee just speeds up the process.
Caffeine stimulates your nervous system: it increases your pulse, breathing frequency and blood pressure. It works like an alarm bell for your entire body to go into overdrive. That also includes your bowel functions. There are plenty of muscles in your stomach and intestines that will react to the caffeine and work harder to get the excrement to its final destination.
Another explanation is simply that your body has gotten into the habit of pooping after you had your breakfast coffee after you've been doing it for a long time. It can be seen like a ritual that your body goes through every morning. Just like night time means sleeping, morning time means pooping.
In addition to the other great answers already submitted there is something called the gastrocolic reflex triggered by warm liquids entering the stomach. Will get things moving if anything is in the chamber.
Nicotine too... Coffee and a cigarette in the morning and evacuation within a couple minutes.
I miss cigarettes. I know everything bad about them, but I still miss them, kind of like a crazy ex. Life’s healthier but significantly less interesting without them. Nicotine addiction was the perfect 10 minute meditation. Forced me to get up every couple hours whether writing a paper at university or working as a young adult. Walk outside, get some air & smoke, and it’s just you away from everything chasing the dragon for a few minutes. Then back to work/school for another couple hours.
Chemicals in coffee (not caffeine) are sigmoid colon constriction agonists. They cause the end of the gastrointestinal tract to constrict, and therefore push their contents forward (towards. The same effect was found in decaffeinated coffee.