What is causing my disc brakes to loosen when it rains?
It's difficult to know for sure, but my guess is that the rotor is getting wet and it and/or the pads end up contaminated, preventing them from gripping as well. You've been solving this problem by tightening the brakes, which works, but it's a sledgehammer fix at best.

Try sanding the pads with a file or some sandpaper. Alternatively, try using sintered pads if you aren't already. This will wear the rotor out faster though.