I've been of the opinion that BG had a lot of variables that he needed to have work in his favor, since early on in this case. I'm also of the opinion that this guy is very observant, and is adept at reading people. May even have a much higher ability to "read people", than most people do, generally. No doubt in my mind he'd been to MHB before, perhaps many times beforehand. Observing adults, observing juveniles, etc.
I think that his going after unaccompanied juveniles was his ultimate prize, and in this case he had the upper hand because the only people there during a set period of time were BG and two girls. Girls he could more easily control than an adult or adults. It makes perfect sense the girls would have stayed together, rather then one taking off and leaving the other behind. This is human nature, and BG knows this.
He forced them down to an area they really couldn't escape from, there was nowhere to go, really. I watched both episodes of the HLN special about the case. Anna Williams says in one episode the girls were not good with directions (paraphrasing). Now put yourselves in their shoes, you're at the end of the bridge, even if you take off running, which direction do you go in? BG forcing them down into the gorge was a crucial thing he had to set in motion, in order to control them and get them on the way to his pre-picked crime scene. He's a predator of humans, and as anyone with a hunting and/or military background knows, you have the upper hand if you can trap animals or humans in a spot they would have to struggle to get, or in this case climb, out of. As Awsi Dooger mentioned, there is probability involved in a lot of crimes, a perp has to take this into serious consideration. He had enough variables to consider before even parking his vehicle and observing people that day, and BG would have had a desire to prevent more variables from being thrown in his way. I believe that by attacking juveniles that day, with no witnesses around who saw the girls after they were dropped off, eliminated variables for him, which I could explain but I think a lot of it makes perfect sense to anyone who has followed the case.
I'm also of the opinion, and people have argued with me about this but I'll let it fly, the girls most likely did not know where the gravel road led to, where it becomes C.R. 625. Which happens to be where Abby lived at the time. Maybe a quarter of a mile or so from the bridge. Again, I can picture myself, put myself in their shoes, at their age. Of course pre-internet and smart phones, but at any rate if I've never been to that bridge before, like in Abby's case, how would I know where I am in relation to anything besides County Road 300, at age 13? Or why would I even care, at that age? Throw in Anna's comment above, and one can see how they would have been at that SE end of the bridge, with no idea which direction to head in walking away from that end of the bridge, which by the way is on private property.
Now picture BG, hanging back a bit on the trail, observing the girls. At some point he notices Abby, who had never crossed the bridge, walking more slowly than the more sure-footed Libby. What would you (WS members) think seeing that? Again, we have to think of this individual as a predator. Two innocent children out for a walk on a trail and an old bridge, carefree and not concerned with much anything beyond the moment, and that beautiful gorge, just talking and enjoying themselves. This was the monster's ultimate moment to strike. He had them trapped at the SE end of the bridge, and like Awsi Dooger mentioned, we don't know what happened to them around that time and between there and the CS. Assault(s), restraints, etc., we don't know.
JMO