It has nothing to do with the water getting hot. It has everything to do with pressure falling low enough that cold water boils anyway.
Water boils when its vapor pressure exceeds the local pressure around it — normally we get there by raising temperature. But you can just as easily get there by dropping the pressure, at any temperature, including cold tap water. That's cavitation: a liquid boiling locally because the surrounding pressure crashed, not because heat was added. Wherever a flow accelerates hard — a pump impeller eye, a valve throat, a propeller blade tip, a venturi constriction — Bernoulli's principle says pressure has to drop right there. Drop it far enough, below the liquid's vapor pressure, and vapor bubbles form on the spot.