I've had a weird history with this. I've always enjoyed the urgency of having to poop. I love the feeling of the turtle head poking out and to this day I love pushing it out and sucking it back in until I can't do it anymore. It gives some release of pressure. When I was around 5, I got this weird feeling I can't describe. It was kind of a "high anxiety" feeling and I would go into a corner and poop into my briefs. It only happened a few times. I don't know why I did it.
Around 15, I started getting a reputation for my farts (my main fetish actually) and friends would always joke that I was going to shit my pants. So I decided to try it when I was alone. I liked taking pictures of it, but I didn't like the feeling. It was humiliating for me and I felt like a complete abomination. I secretly liked that feeling afterwards, but not the feeling of pooping in my underwear.
In college, still with my reputation as a prolific farter, same thing happened. I said I was going to fart until I shit my pants, as a "joke." A few accidents had happened around friends before. I was with two other friends this time. One of them then dared me to do it in my underwear and he'd give me $50. Well, I did it and they were both in shock but cracking up. My friend ended up not having the money and didn't want to go to the ATM in the rain. I told him he could keep the money if he'd get on the floor on all fours like me and do it too. He actually did it. He didn't enjoy it as much. I wrote up the full story on Reddit. The other friend who didn't participate was disgusted and said we needed diapers lol.
After that, I was hooked. I do it a couple dozen times a year and usually record it.