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The substrate is a very thin piece of glass. If you were carrying it in a backpack, then something else could have squeezed it or a shock when putting the bag down or bumping it on a door frame. The initial damage could have been a few days ago, and then thermal stress (heating expansion/cooling contraction) caused it to spread -- hard to say without having followed you around.
If you want a reader that's more durable, the newer flexible plastic Mobius substrates are more durable, but I think they're only available in the larger readers.
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