Few years too late here, but I always found this to be the case when ever I enter my garage. I thought it was always just bad timing, even thought just before leaving my house to go into my garage (not connected unfortunately) that I didn’t have to go #2. But I looked it up and I believe I found a reason. This is just my hypothesis, but it might be the same reason for you as well. My favorite place at home is the garage. Even though it’s messy, I love the smell of gas and oil and I love tinkering. And it’s cozy in here, in a dirty, industrial kind of way. And every time I enter I always get a shiver down my spine due to a dopamine release and euphoria of being in my favorite spot, and a sense of relaxation because I’m in my own space. And according to a google search, such quick spikes in euphoria as well as relaxation, stimulates the whole body. Including the digestive tract. So your body thinks it’s time to poop.