I think Libby began recording BG when she started feeling suspicous or uneasy about his presence. I think BG was approaching the girls rapidly, and with nobody else in sight she felt in danger. When she realized that BG was a threat she put the phone in her pocket/bag/bra/whatever to hide it, but left the recording on. I think that BG grabbed Abby and possibly drew a weapon (such as a gun or knife). I imagine he would have said something to the effect of, 'come with me or she will get hurt'. Libby may have then said, 'where', which prompted the 'down the hill' response. LE mentioned that Libby captured audio of criminal behaviour about to begin (or something to that effect). I don't think they would release any audio or video of the girls in distress. And I'm guessing they were in distress as soon as they left the trail.
I think BG might have strongarmed Abby off the trail, and threatened both girls with a weapon. Libby could potentially have run off, but complied partly out of fear, and partly out of loyalty to her friend. Maybe she thought (assuming only one attacker) that together they could fight him off. I'm guessing there is audio of their entire abduction but it would be inappropriate to release to the public. The 'criminal behaviour' was evident as soon as the girls went 'down the hill'.