Why did my solder joint fracture?
Looks like a cold joint to me- the slightly crystalline appearance, for example (though it's hard to tell with all that flux on it). It was probably fractured from the beginning and corroded a bit so it stopped working. 

You can avoid this by preventing motion as the joint cools. You can catch such defects by inspecting the board with a 10:1 magnifier, and to some degree, by burn-in at elevated temperature.