An easy way to visualize what is happening with the air bladder of a fish can be demonstrated with an upside down glass in water that is filled with air. When this upside down glass is just below the water surface, the glass will be completely full of air. When this glass is lowered to 10 feet deep, the water pushes on the air in the glass so the glass is only about 90% filled with air. When this glass is lowered to 50 feet deep, the water pushes on the air in the glass so the glass is only about 20% filled with air.
Imagine that you had an air tank under water and filled the glass full of air at 100 feet deep. You bring the glass toward the surface and with each few feet upward some air would escape from the glass.
Imagine taking the same glass back to 100 feet deep and filling it full of air. Then stretch a balloon over the end of the glass. As you bring the glass toward the surface the balloon will continue to get bigger. This is what happens to the air bladder on a fish that is pulled up from deep water.
The fishes body acts like a minature air pump and can slowly adjust to depth changes. However, it the fish is pulled from deep water to the surface before it can adjust to the pressure, just like the balloon would get bigger and burst; the fish’s air bladder would get bigger and burst unless enough air bubbles could escape.