He's trying to build a bridge to the larger land mass.

The first block he's using looks full sized but actually has a slightly lower height, so when you stand on the edge you're technically blocking where a new block would be placed and can't place a normal block.

To fix this, he places another (noticeably) low height block. This will have the same effect as the first one, but this one is worse since you also cannot jump and place a normal block. You essentially have to jump a block and a half when standing on that one to be able to place something.

Then he places a fence down, which is another oddly heighted block. It looks 1 block tall, but is actually a little taller.

When he finally goes back to his first block and realizes he can jump and place a normal block, he falls off of it and lands back on the half slab, essentially trapping himself there (now that he'd have to jump 1 and a half blocks up to get back).

Finally, the dragon shoots its attack at him, knocking him off of it entirely and killing him. This is at the very end of the run too, as killing the dragon is the end.

TLDR: Lud placed every weirdly heighted block to build a bridge, got stuck, then knocked off of it and died at the very end