Re: Tooth chipped for no reason...
It's possible that over time, you developed a tiny fracture in the tooth from normal wear and tear on the teeth. Because the fracture was not into the dentin, (soft part of the tooth) or close to the nerve, you wouldn't necessarily feel the tooth cracking. These tiny fractures can break apart at any time.
The front teeth are more thin and fragile than molar teeth. You may have taken a bite of something months ago or just general stress on the teeth that triggered the tooth enamel to begin to fracture.
Decay is a possibility, but this reads more like just a tiny fracture had formed and split part of the enamel causing it to chip.