Well, here is my current thinking for a female in a multi-perpetrator scenario. Libby was concerned enough to start recording video. BG was quite a distance away. We have gotten to see a clipped and enlarged (effectively zoomed plus some other adjustments) view of BG. Libby and possibly Abby would have seen him differently and certainly not as well as we see in those not so good images. There isn't any indication of anything in BG's possession threatening the girls from that distance - not in his hands for sure since we can't even see them. Also, if Libby was concerned enough to start a video I would think that level of concern would be enough to make them want to get out of there. Yet, that does not appear to be what happened. For some reason the girls could not get out of where they were.
So, my working theory is that there were one or more perpetrators near or behind the girls and that meant they were effectively, if not actually, trapped in place. If there were another perpetrator and it was a lone male that was near or behind the girls that would likely be enough to raise the concern to start recording. But, given that there aren't dense woods in the immediate area of where the girls were (per GH) at the south end of the bridge and no leaves or foliage then their view of the immediate area would be pretty good so it would not be as if someone could easily sneak up on them. However, if that other person were a lone female or a male/female couple then that might not raise the alarm that caused Libby to start recording until something started happening closer to the girls. Either way, I believe something occurred prior to BG getting anywhere close to the girls and that is what caused LIbby to start recording.