The waves are produced by mass accelerating through spacetime, so in theory everything produces them—you, me, a black hole, a neutron star. As physicists learn to tune their instruments, more sources of the waves will be within reach. But you're right: the odds of detecting a signal this strong and clear right off the bat were low. If LIGO hadn't happened to be on when the waves reached it, after 1.3 billion years of traveling through the cosmos, they would've been missed completely. So this was a mix of extraordinary science and extraordinary good luck.
