I keep thinking, and have mentioned to friends, that I think the guy is quiet/reserved and normally very non-violent. Someone with social phobia (among other things) who almost never speaks to strangers. Based on his clothing (inappropriate for the weather) and walking along the side of the bridge rather than down the center, I think he may have some other odd quirks which make people somewhat uncomfortable around him. I keep seeing a family member laughing and saying, "Hey, that almost looks like you!" but never suspecting that it really is.
I wonder if the girls were taking pictures on the bridge, and videoed him coming across the bridge while they were out there. It looks like it was taken from quite a distance, and with him looking down, he may not have noticed that he was being recorded. After he reached the south end, they may have immediately started crossing to the north side.
Being the fun-loving girls they were, they may have been laughing and joking around as they crossed, and he could have assumed they were laughing at him. It may have been anger pent up from teasing all his life combined with being in a place with no witnesses that made him decide to assault them. The only problem (well, one of the only problems) is that he sounded so calm in the recording, and he would have most likely just beat them right there rather than leading them to where they were found.
Just trying to come up with something different, and even more so, something that doesn't involve sexual assault. MOO