What is causing my disc brakes to loosen when it rains? I have Tektro Aquila disc brakes, and I have recently noticed that the brakes need tightening every time it rains. I don't see mention of this model in Are there any major outstanding recalls of bicycle parts?, nor does there appear to be a recall for Tektro brakes.

The brakes are about four months old, and have about two thousand kilometers on them, plus the barrel adjuster on one of them is most of the way out, so I suspect brake pad wear is a factor. I'm going to swap out the brake pads, but is there something else I should be looking for?

EDIT: I got done changing those pads and one of the worse offending pair was literally worn to the backing plate.
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.