The time of day and it being a weekday doesn't fit, but I've often wondered if BG maybe had a girlfriend in Delphi, possibly a few years ago. It could be that BG and his girlfriend went to that area to be intimate, maybe the girlfriend was married, or maybe it was just exciting. BG could have gone there in the hope of coming across a couple and quite possibly would have killed the man immediately and terrorised and killed the woman. The alternative is that he knew children weren't going to school that day. Somehow he knows that teenagers will go to that area unaccompanied, so it was worth a trip out to try his luck.
Either way, he may have been stalking around for several hours and becoming increasingly frustrated and even angry, as there were several adults there. Finally he spots the girls, this may not have been his planned quarry, but he's frustrated and he needs a 'kick'. He knows there's other people around and although they may not have seen him yet, the chances of being seen with the two girls is very high risk. So he's quick. He gets the girls under control (a gun/knife, threats to hurt one girl etc) and gets them to a 'safe' and unseen place asap.
I just think his past may lie in sex crimes. He started off as a peeping Tom and then moved into rape. He is undoubtedly a really horrible and frightening sexual partner. I think it's a wife or long term girlfriend who is the key to this. I hope to God they join up the dots sooner rather than later.