IMO, since it is not much of a real hike and the distance is so short and they had 2-1/2 hours or so to be there, they could kind of wander around and just sort of hang out. No real need to even walk the whole time. They probably stopped several times and just talked and laughed or whatever.
So that was why I wondered if they tried to hike to the left of the north side of the bridge and saw something... and were seen... so that BG came back from the NE side of the creek and then onto the bridge to confront them.
Maybe they trespassed or maybe they saw him doing something he shouldn't have been (drinking, peeing, exposing himself, whatever...) and they laughed and ran back onto the bridge and then at that point worked their way across the bridge and thought no big deal.
Then when he started walking towards them, their radar went off and that's when she started recording.
Something had to trigger her want to record that of him.
Again, this is entirely speculation with no facts to back it up.