Fermentation. If bacteria and yeast are able to get into the sugary part of the watermelon (and some can without any hole in the skin or anything), then they will do what bacteria and yeast do: eat sugar and poop ethanol and produce CO2 as a byproduct. Then like any other sealed container that has a gas released as a result of a chemical reaction, once the pressure is high enough, it bursts and the pressure is released rapidly in an explosion.
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