Water below 0°C does not necessarily turn into ice at once. At normal pressure, 0°C is the equilibrium temperature for liquid water and ice, but freezing still has to begin with a sufficiently stable microscopic ice nucleus. A tiny embryo creates an ice–water interface; if it is too small, the energetic cost of that interface makes it disappear again.
When water is relatively clean, its container smooth and the liquid undisturbed, there may be few dust particles, bubbles or ice fragments that help nucleation. The water can then remain liquid below zero: a state called supercooling. Tapping the container does not shake extra cold into it. Instead, the disturbance may create bubbles, alter a contact line or expose an existing particle as a nucleation site. Once a nucleus crosses the threshold, neighbouring molecules rapidly join the crystal lattice and branching ice appears to sweep through the liquid.
The display is quick, but the whole bottle does not usually become solid instantly. Freezing releases latent heat, warming the remaining liquid towards 0°C, so the first result is often slush. The phenomenon shows that a phase change depends not only on temperature but also on whether a pathway exists for the new structure to begin.
https://van.physics.illinois.edu/ask/listing/19505
https://link.aps.org/doi/10.1103/PhysRevE.97.023103
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