The mechanism hasn't been settled. Evaporation, dissolved gases and thermal momentum have all been proposed. One of the latest ones has to do with the structure within water. At colder temperature, the water is more hydrogen bonded but in shapes that are somewhat different than in ice. To form ice there needs to some rearrangement. In the warmer water, the structure is more open so the molecules can rearrange faster into ice.
A loose analogy is two people building a card bridge. One is starting with a deck of card and the other with a card house. The house looks closer but we got to carefully remove some of the cards to make this into the bridge. By then the person with the deck is catching up and finishes first.
Plato proposed water was an icosahedral. It turns out he was right in the sense of hydrogen bonded structure, which turns out to be more common in the cold water. The figure show a transition between two states.
