As water cools in the freezer, at first it starts to shrink. But when the water molecules get organized and crystallize, they take up more room, and so the water rebounds and expands. This is why glass containers break when water freezes inside them. In the star, after the initial compression of the neutron core, there is a rebound effect less than a thousandth of a second later as the neutrons arrange themselves in a crystalline structure which suddenly expands.
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