When I heard about this, I decided to head up to the Vinewood hills and jack myself an Obey 9F myself to investigate. I drove around the area a few times and after about a real life minute, I heard a similar screaming sound coming from the dock, however I didn't witness a pedestrian falling or anything like that.
If you had to ask me, I believe this is a simple pathing error. The boardwalks around the beach, including the Pier, are no-vehicle zones, so as you are driving it probably scares away the pedestrians. Now, video game characters are pretty stupid sometimes, so instead of running down the dock, the frightened NPC will just run towards the nearest path, which just so happens to be on the beach or sidewalk below them.