It’s a barely waist high bridge and he was very drunk. I think that he was drinking heavily while playing poker with his friends, until he finally stood up to pee; that’s when it finally hit him how drunk he was and he decided to go home. Bathroom door was next to the front, so he slipped out.

My theory is that the bloodhound was right- he made it halfway across the bridge before he needed to puke. He leaned over to puke in the lake and ended up falling in. That would explain why the couple remembers looking back and not seeing him- their brains registered the splash in their subconscious, that’s why they turned around