Must have been a massive leak. We see hydrolock pretty regularly, but it is from water coming up the exhaust in marine application during panic stop. Forward speed of the boat, operator quickly brings throttle to neutral, pauses a second, then jambs into reverse (to help stop, they think). Unfortunately, the forward momentum with a prop in the water, now in reverse gear, can/will spin the now idle speed engine backwards for a split second due to the prop force being greater than the torque of the engine at idle speed, now exhaust (which is under water) becomes the snorkel directly into waiting cylinders. The boat will stop much faster bringing the throttle to idle and leaving in fwd. The forward momentum is coupled with the prop and engine - engine braking.