Why this happens? Especially with heavily used crystal tools, you may encounter surprise shattering when the crystal has absorbed too much energy. In essence, it is done. Sometimes it has not been cleaned enough and can't hold too much of a negative charge. Sometimes it has just drained itself of its own energies. Sometimes it's just physical. Whatever the case, thank it and lay it to rest.
Another idea I have been using recently is placing these pieces in my artwork. It can not harm you because it is not being used actively in healing, but the crystal is still giving off a pleasant, natural vibration from an admirable point of view.