Not knowing the reason for something is a "reason to keep it" -- well, it's a reason to not do anything. If that something gets destroyed by, say, a force of nature, would Chesterton's Fence tell you to rebuild it? No, I don' think so.

Force of nature is misleading in the context of where it is likely to be applied. No social norms or institutions subsist without maintenance. But let me keep it and tweak it a bit, if you could easily prevent the force of nature destroying the fence, would you say the argument encourages you to do so?