When the rod bolts came loose (two bolts that hold the cap on the rod, which holds the rod on the crank), it allowed the rod bearing to spin. When that happens, the bearing will continue to move between the rod and crank, and it eventually wears the bearing out. That causes a lot of slack and lets the rod move on the crank. When it moves, it will hit the block, and eventually break.
Typically, you can hear them knocking. But its a freak thing when your rod bolts come loose. Thats why the engine showed no signs of going down. It just happened. Those bolts came loose, and the rod started moving on the crank, and that caused the bearing to spin, and then all that slack let the rod hit the block.
I'm lucky it broke where it did, because if it would have broke say in the middle or top of the rod, then it woudl have throwed it through the block instead of the oil pan, and more then likely, instead of just bending a valve (happened becuase when the rod broke, the piston shot up in the cylinder, and hit the intake valve), it would have broke the valve, and then shoved it through the head. If that would have happened, then the entire engine would have been junk.