Aliens. That’s such an interesting explanation. But if I were scared of large flat land, it would be because of the lack of anything “safe.” There aren’t any buildings or houses to enter, there aren’t any trees to take shelter under, and there aren’t even hills to hide behind or valleys to hide in. If a dangerous situation were to occur, like an attack, violent weather, etc. you would be at a disadvantage.
Also, another explanation is the inability to see an end, if you will. If you wound up in the center, there are no big landmarks to show you the way out. There would be no indication that it was even possible to get out. For all your subconscious knows, the field could be infinite. You’d most likely be lost. And that would be scary.
Of course, I say these things as if they are valid concerns. In a typical situation, if you’re careful you’ll be fine. But some things are too deep seated to change, and while I encourage you to try and overcome, I don’t blame you if you choose not to. As long as it doesn’t interfere with daily life and normal functioning in society, you don’t have a disorder/phobia.