What does it mean that the shark was not an accident?

The shark didn't accidently come upon the boat, it had gravitated toward the cloud of blood.... the marlin's blood.

The shark was not an accident. He had come up from deep down in the water as the dark cloud of blood had settled and dispersed in the mile deep sea. He had come up so fast and absolutely without caution that he broke the surface of the blue water and was in the sun. Then he fell back into the sea and picked up the scent and started swimming on the course the skiff and the fish had taken.