Pardon the dust as I clean off this old thread.
I don't think there's a direct connection between loving the Lego Castle theme and having a higher education degree in history, any more than I think you need to be an astronautical engineer to love making space MOCs. (Admittedly, I do know the guy who designed the Curiosity
rover set. Real-life NASA engineer. He's a member of my local LUG.) It's easy to be an enthusiast, even an expert enthusiast, without having your knowledge certified by academia.
In my case, I'm an editor (both fiction, which I prefer, and nonfiction, which I get paid more to do). I do have a degree in history, with a focus on intellectual trends as demonstrated in fiction and mythology (aka studying the stories cultures enjoy as a way to study said cultures), and that probably helps me, but you couldn't tell from any history papers I've ever written, much less published. You could much more easily tell it from my lectures on creative writing, at least when I get into worldbuilding techniques and the importance of research.
Sometimes you'll see connections, like what have already been pointed out here, or people like Mr. Red, a frequent con-goer in the US and Canada who's a history teacher and likes making impressive historical displays. Most of us just really like it as a hobby. And I think that's cool. I can get stodgy about accuracy sometimes, so I've occasionally needed a reminder that it's a hobby and accuracy doesn't matter as much as just having fun.
At the same time, I've also noticed that once I joined the local scene some years back, discussions of accuracy started having an effect on what was displayed. I knew a lot about what medieval life was like, and when I explained some of the choices I made in designing my displays, a few others started taking a closer look at historical details. My display is frequently focused on what the minifigures are doing, rather than on a static display. I didn't intend to tell anyone what to do, but it's really cool to see how we all influence each other. I've been influenced by so many others that it's a bit hard to believe that I've influenced them back.