I have always thought the girls had a previous run in with this guy, particularly on the specific day at the trails. If the latest info that has been reported is true, which is appears to be so, the theory of him entering from the SE end of the bridge doesn't convince me now. Still possible absolutely, but I now firmly believe this guy was somewhere at the North point of the bridge, possibly waiting on the trail already. Something occurred, whether he spoke to them before the bridge or was just acting weird/off. He was weird enough for Libby to start recording him non-chalantly, and was probably going to show someone later, this creepy guy from the bridge.
All just speculation of course, but I am one to think the answer that makes the most sense is probably the answer you're looking for.
I also belong to a group on FB where BP posts regularly to confirm or deny evidence/rumors. Last night she confirmed listening to 20 seconds of audio, if I am allowed to post screen shots from these groups please let me know and I will do so! Just don't want to break any rules.
Today is the day.