This happens commonly on vehicle/motorcycle tires, and for similar reasons. Non-bicycle tires use woven metal 'bands' that form the structure, with rubber molded around them. Bicycle tires use cloth/nylon/some other type of non-stretchy material with the rubber molded around it. You can usually feel them as the lines inside the tire.
When bands break and/or separate, it allows air pressure to push out the now unsupported rubber around it out farther than normal. You may not even notice the inner part following, because they're not really one piece, more like two pieces of bread surrounding the meat (the bands). Further, they don't need to make both sections airtight, so usually just the outside is (even if you don't see holes, the rubber can be quite permeable (indeed it usually is to a small extent, even airtight ones).
If you're asking what causes the breaking, it could be poor manufacturing, extreme flexing, folding, extreme heat, sitting flat on the bead for a while (or getting ridden on while flat), over-inflation, or any number of things that stress one section of the tire more than the rest of the structure.