Part of me has been enjoying the recent finding out going on. Maybe it's because properly linked and referenced sourcing is cool and saying where someone got an idea doesn't devalue how they're implementing it. Maybe it's because the people who are working on projects similar to my own are colleagues and not competition, since we all do better when we all do better.
With how many billion people in the world, any system that expects a monopoly on an idea is a bit suspect. Very few if anyone has a truly unique idea. It is more about who claims an idea most convincingly and effectively.
My impression is it wasn't so much about thinking they had a unique idea that the others were "stealing", as fearing the other books with similar themes releasing at a similar time would pull attention away from their book. Which is actually a bit silly, because generally the first thing someone does after loving a book with a particular genre/theme combo is go and look for more of the same. Cooperating with those other authors, so they all plug each others books ("Hey, if you loved my book, go check out X, Y, and Z too!"), would have been so much more productive.
Readers who like a certain genre tend to read multiple books from the game genre, so these authors should be more like friends instead of competitors, which makes this betrayal harder to understand.