Im sure this has been discussed before, but Gray Hughes presented a theory, that isnt genius, but laid out in a way that really seemed possible/probable.
He suggests Abby and Libby werent far on the bridge at all when they see BG approaching them. They keep looking back and he is still coming, and still coming. And youd have to wonder, being a kid on that bridge, why is he still coming this way? Why is he so motivated? Why is he so motivated? He's just hustling across and it must have been freaky. So Libby pulls out her phone, keeps it down low (no it was not selfie mode) as he keeps coming.
Gray then speculates that BG is holding a gun in his hand, under his jacket, and soon after the released video ends is the "Guys". Then BG walks past them at the end of the bridge. Libby is at the end, Abby behind her, because she is scared, he steps behind them after saying "hey guys" then he pulls out the gun and says "get down the hill." Then he aims with the gun the direction to go, and they go. And he is sure the camera is still recording the video which would only be useful audio at that point.