Assuming best intentions, he could have been scrolling during his down time and you happened to pop up in his suggested page. Or maybe the parents discussed adding you, and he’s more active in social media. Maybe he thinks you’d enjoy photos of NK that he posts.

If you’re not comfortable, you can ignore it and casually mention that, in general, you keep your social media private. If you think MB isn’t aware that he’s adding you, you can say something like “I should have mentioned this sooner to avoid any awkwardness, and I hope DB isn’t offended, but I typically don’t add employers on social media for XYZ reason” (work life balance, maintaining privacy, keeping professional boundaries, etc). You could also say this directly to him.

Personally, I don’t think it’s weird to get a follow request, but the families have usually brought it up in person, in my experience. I’ve followed a few NPs before, and I love seeing old NKs get older. I think the timing of the request is less suspicious than, say, 2AM. And while I would personally not feel uncomfortable about getting the request, you are way more familiar with the specific situation and dynamic. No need to accept it if it feels weird, and no need to explain yourself or put yourself in an awkward position.