TL;DR - Planned C-Sections. There's also a drop on weekends for the same reason as fewer individuals plan for a delivery on the weekends as hospitals are staffed with fewer people.
I've seen similar patters before. I assume the famous "things break on weekends" is because of somethings similar, ex. because there are fewer people to look for and fix small problems before they become bigger and make things fail.
I also never publish websites on Friday, although for some reason most of my clients want to start the weekend knowing that they're done with it and always beg me to do it.
You just want to play safe and make sure if something can go wrong you're ready to deal with it—in this case by having doctors around rather than being 3AM and the hospital is half empty.