I have had the same problem too. I use hollister and it happens down at the tail. They are small and kind of like bubble wrap. Of course being the person I am I have popped them
and believe it or not they didn't leak. I know I probably shouldn't have done that but I just can't leave my hands off of anything, LOL.
I did a little research and it seems that most major companies use a process of "stacking" multiple layers of thin plastic to achieve a stronger, thin plastic bag. The bubbles in my situation seem to occur somewhere between two of these layers. The only thing I can associate this phenomenon with is me using peroxide as a deodorizer. When I tried M-9 I had no bubbles, now that I am back to peroxide, the bubbles are back, not every time but I do get them. There may be other sources for the bubbling but for my case it seems to revolve around the peroxide. Not sure if any of this will help but at least you can see you are not alone.