Mine popped off this week, after a launch, (Reo), aswell. I have found that if the donkey is a little long for the spreader bar and hook then it tends to push a bit against your body, and pop due to it being pushed. This happened also frequntly with one of my chicken loops for one of my switchblades as well. The harness i am using is a mystics and also have a dakine, it only happens on my mystics, as it has a shorter hook.
KR