Virtually anything.
Hit it with a hammer, drop it from a plane, or glue inserts in it and throw it at pins.
Cold weather, hot weather, gravity, cosmic radiation, anti-matter, syzygy, inner core expansion, cover shrinkage, bad luck, black cats, the strong force, static electricity, dark matter, seismic irregularities, quantum mechanics, harmonious sub-sonic vibrations of the nucleus of a neutron star, or fate.
Its all the same. The damn thing cracked, and you gotta get another one.