How did bubbles form on my tire? I was riding recently and kept hearing a noise similar to a rock stuck in a tread, but I have no tread.   When I got off and inspected the tire, I found these weird bubbles had formed.   I have replaced the tire (the bubble doesn't telegraph through to the inner wall), and the tube appeared just fine.   I can't figure out what caused this though.  I'd really like to know, so if it's something I'm doing, I can keep from doing that.   

It was my rear tire, and the tire only has about 1200 miles on it.
I suggest taking it back to the shop you got it from, as it looks like a manufacturing defect. Since the bubble isn't visible on the inside it's clear that the layers of cloth inside the tyre have separated, and that's not supposed to happen. You should be able to get a warranty replacement.

Sheldon Brown has an explanation of how the tyre is made that may help you picture what's going on: http://sheldonbrown.com/tires.html