Looking at this graphic (if I am reading this correctly), the girls were more or less at the end of the bridge when the footage of BG was taken. It seems like a very long bridge so I would assume that they would have known he was behind them for some time?
Again, looking at the graphic, which way did they go? Because the distance for the end of the bridge to the water appears quite far, which is why I'm curious as to where he took control over them. Did they leave the bridge and decide to walk back to the pick up point and were approached in the forest? Abby may not have been familiar with the area, but Libby and her family were.
According to reports, one of the girls father, Derrick, made it to the bridge, but turned away rather than crossing. The elderly gentleman that he spoke to mentioned that he had only seen a man and woman under the bridge. Where are these witnesses? When they were searching for the girls, they were calling their names and it is said that the voices carried very well. If the girls were close to the bridge and perhaps they screamed, then the people under the bridge should have heard something.
ETA: There was a female hiker on the bridge taking photos at the time the girls were murdered. Why did she not hear or see anything? Why did BG not target her or know she was in the area?