To explain, I must get a little technical. When the filament in the bulb breaks, an arc sometimes forms. This arc often expands until it is across the entire filament. Usually when this happens things get a bit unstable, since the more conductive arc would draw even more current. The arc easily becomes conductive enough that it draws a few hundred amps of current. At this point, the arc often melts the parts of the filament that the ends of the arc are on, and the arc glows with a very bright light blue flash, and at the same time, causing the fuse to burn or breaker to trip. Many good quality bulbs have a small built-in fuse inside the base that would normally burn before your breaker has a chance to trip.