The answer is differential expansion, as Martyn Poliakoff explains below. The outside of the ice cube is warmed by the drink and expands, but the inside remains cold. The stresses thus introduced cracks the cube.
Interestingly, you can make this happen the opposite way, as well. Drop some ice into liquid nitrogen, and the outside is cooled and shrinks, while the inside stays warm, leading to a similar fracture effect.
Check out the full explanation in the video below. --Ryan Cooper
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