Minor ripples in the water will cause those plates to move around and collide with one another. I know this sounds silly, but it's basically the same concept that causes earthquakes. These plates are so big, they actually cover the entire surface of the planet. Even so, there are still locations where these plates come together and collide with one another. Scientists call this place a fault line.
Tectonic plates move very slowly, only a few inches every year.
Yet the collision between plates can be very dramatic. Earthquakes are common symptoms of plate movement. They occur when plates collide but neither one wants to yield the direction it's going. Take the San Andreas Fault for example. The San Andreas Fault is responsible for almost all earthquakes that occur in the California area and is the fault line between the North American and Pacific plate.