I agree with this.I think they saw him expose himself to them and he probably said something like " do you want etc" like he did to a previous woman. He followed them at a distance across the bridge and they started to video him and record as he ordered them down the hill, where they fought, ran and he pursued them , shot and killed them burying them under leaves (as he had seen his mum buried). He left via the easern side of the creek and rejoined the trail.
AJMHO
All just my opinion:
I have always had a feeling about the timing of this crime, and that it most remided me of shoplifting - in the sense of seeing the shop momentarily unattended, and taking immediate advantage of the opportunity.
Whether it was DN or another person, I think that at least Abby tried to walk past him, then both girls tried to get away so he used a weapon to march them, I think they tried to get away again and he killed them for it, with or without SA.
The one BG photo on the bridge where is crossing his foot over, has always looked to me like he was blocking Abigail from passing by him to go back over the bridge. This blocking step would be where the crime commenced, and to me explain her shirttail leaving the frame as she tried to rejoin Libby at the trees.