It has happened to me twice - the first time I was giving people at work a test drive at lunch. Like you, from a dead stop with moderate acceleration, the gate dropped - sounded like the transmisson was falling out. I passed it off as not shutting it well when giving the tour.
The 2nd time, I let my father-in-law take the wheel. Same scenario, the gate dropped - should have seen his face - he too thought he dropped the tranny.
Since then, I have made sure to place my hand on each end of the gate, shutting it firmly, with moderate to strong momentum. Before I was shutting it with both hands near the middle. In addition, I lock it now. Hasn't fallen, but then again, I have not had the chance to drive it much because it is in for leak repairs