If that's how you hung it, and you used the standard JoAnne's ripstop (and that the ripstop there is roughly comparable to the one near me...oldgringo, as usual, has a very pertinent point about changing stock), I'd say that one of three things likely happened:

1.) Something in your pocket or on your person abraded it. A knife, a rivet on your jeans, your car keys falling out of your pocket, even a metal clasp on the back of your hat, all could have caused a tiny tear somewhere in the weave. Put weight in it, and...

2.) You hung it too low one time and brushed something on the ground. As above, except from below.

3.) There was a flaw in the material. Not all ripstop is created equal, and the grid in the ripstop isn't meant to stop a tear with your body weight behind it--it's meant to stop smallish tears with the weight of a garment behind 'em. Given a small flaw in the fabric, well...

Best guesses, anyway. I could be wrong (again), but...