Hard to say when a fastener is going to loosen up for whatever reason. Anyway, I'm pretty sure it came loose because the bearing was basically seizing up. If both the water pump and the fan are spinning the same CW direction (when looking at the front of the engine), any time the engine RPM increases there will be a counter CCW torque from the inertia of the fan going through the inner race of the bearing and therefore on the bolt, which would try to unscrew the bolt. Having that counter torque applied over and over enough times is probably caused the failure. When the bearing is working as it should, the counter torque isn't much (just the internal drag of a normal working bearing), but with the bearing seized up that counter torque would be much higher. It was essentially was like a "mini impact" gun on the bolt, and took some time to loosen the bolt.
