Acoustic Guitar Bubbling Under Bridge. Help!

I own two acoustic guitars. One has a very noticeable bubble under the bridge and I believe the other is beginning to bubble as well. Can anyone tell tell me what could be causing this? The fact that it is happening to both of them means it is probably something I am doing wrong. Could I be stringing them incorrectly? I have heard that heat will do it, but that can't be the issue. I haven't had either of them in any extreme heat (or cold for that matter). Please help. Thank you.

I would guess that it may be humidity-related. It doesn't necessarily take extreme temperatures. If it's cold where you live and you're running the heat in your house, even at a comfortable level, it can dry out the air and cause problems, particularly with acoustic guitars.

That could be it. I do live in a humid climate. Is there any way of preventing it if I got another guitar?

Either use a humidifier to control the humidity in the room where you keep your guitars or keep it in the case when not playing and get a case humidifier.

I've also heard good things about the Planet Waves Humidipak, which is supposed to keep the case at ideal humidity, but I have not tried it personally, so I can't say whether it works or not.