Thanks to all for the great feedback. Everyone, including the neighbor and deputy agree that there was no possible way the rounds would do this except as ricochet. Using topo and aerial maps, we've estimated that the rounds would have had to travel 200 yards past the backstop, strike a tree, change direction at a 60 degree angle, and then travel another 250 yards. After hearing many "guess what happened to me" stories about richocets, I concede that it is possible. What I find unlikely is that nearly every bullet we fired, took this same path.
I have torn down the earth berm and instructed the family that the "range" is closed. Going forward, however is the task of rebuilding the "range" in a safe manner and avoiding this situation entirely in the future.
I have not attempted to prove/disprove my theory about the sound waves from the gunfire "sounding" like something in the trees. I hope to do so for my own satisfaction at some point, but now is not the time. Most of my neighbors are horse people, not gun people. There have been incidents prior to my purchase of the property that left negative emotions about gun people. A neighbor on the other side of the ridge, 800 yards away from my property line, had a round during the 2012 deer gun season, strike his house. Of course, my incident and this occurrence have nothing to do with each other, but still they are associated and spoken about as though they are related. My only defense is to make the new range as safe as is economically possible and insure all family and friends use safe firearms practices.