There's a name for this very common occurrence: the Mariko Aoki phenomenon. The name comes from a woman who mentioned it in a magazine article in 1985, but it quickly spiked into a TikTok trend, when many users raced to the comments to say, "WAIT. ME TOO." And also, "why???"
To get to the bottom (heh heh) of this, we spoke to Nurse Susan Wong from Butt Talks TV, who has over 20 years of experience in pelvic floor problems and has helped patients diagnosed with conditions that affect the anus and rectum.
"In the colorectal world — where I spent most of my time at the University of California, San Francisco — it could be related to the person finally 'slowing down,'" Nurse Wong told BuzzFeed. "For instance, sitting to read a book, or perhaps the sitting position on the chair. This could stimulate someone to feel as if they needed to defecate. The calming environment probably was helpful too. Along with the possibility of having some other triggers — like having a cup of tea or coffee before going into the bookstore or during her visit to the bookstore."
Nurse Wong also agreed with Dr. Sameer's finding in the Men's Health article about this same phenomenon. In it, Dr. Sameer Islam, MD, a Texas-based gastroenterologist, stated, "In a library or bookstore specifically, what's likely happening is the effect arises from feelings of nervous tension in the face of all the information represented on the bookshelves. This has more scientific support and evidence, at least. But we really have no idea."
"The mere position of sitting, bending forward to read a book could have also stimulated and simulated the perfect setting for the gastrocolic reflex to react, hence having a bowel movement. People may have repeatedly experienced this and associated the 'bookstore' as the trigger, rather than all of the other factors that I have mentioned before."