The above waist high railings are as good as I've ever seen erected in Thailand to protect earthworks.
In past years I've seen video of people duck diving in an unprotected, flooded hole in Patong to retrieve motorbikes completely under water. I remember a woman dying when she drove her car into a cutting across a street that was only protected by a tree branch on the road,
Who knows what this guy was thinking or trying to do when he climbed over the barrier. There was no foul play, and he didn't trip over anything, He climbed in as shown on CCTV.
Whether he was drunk, drugged or not can only be confirmed by an autopsy.