Well first of all it shouldn't reverse rotation, however it is possible with a misfire (engine firing out of time). If you consider conceptually how an engine works during the down stroke of the piston it creates a suction into the cylinder typically for the purposes of bringing in fresh air and fuel mixture. The valves of course are timed to open and close with the stroke of the engine. So as you go through the strokes the engine will accomplish in one complete cycle, piston goes from the top of the stroke and proceeds down, the intake values open and allow fresh air and fuel in (or the fuel injectors fire at that time) as the piston bottoms out the intake values close and the compression stroke begins where the piston moves up compressing the air fuel mixture till the top of the stroke where the spark plug fires and thus results in the power stroke where that explosion pushes the piston down with force as it burns and expands, finally the piston will take one more travel up and as it does so the exhaust values will open expelling the exhaust. It's at this time if there is a misfire it is possible to actually suck water back into the engine through the exhaust valves. Why? Well the exhaust outlets actually contain water at a stand still, this is used as a muffler of sorts and creates back pressure for the engine. Depending on how that is designed, and some were poorly designed, with little effort the potential reversal of the piston with the exhaust valves open could allow water to be pulled into the cylinder and thus create the environment for a possible hydro-lock.
One other condition that can do this is if the boat is travelling at speed and the engine conks out, the back wash at the transom as the boat loses speed and comes off plane can create enough of a back flow of water to force it into the engine even if it is not moving.
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