Uhm, you're not the brightest bulb in the pack are ya?
Water pools up in teh combustion chamber. Probably more so on one rotor than the other. When you crank the engine, this water gets thrown around. Just like a rotary flooded with fuel loses some compression, the rotor flooded with water loses some compression. IT also gets thrown onto the leading sparkplug for that rotor, maybe even the trailing plug. When the opposing rotor starts up, this one fails to combust, because it's flooded with water, and the plug even has water on it. After a few seconds, you increase the engine's speed, creating heat, friction, adn compression, all of which expel the water...then the rotor picks up and begins firing with the other one.