I thought it was a bug, but then I realised this is a thing youtube are doing and it's fucking awful.
Edit: double checked and because they moved the layout the Unhook extension isn't working properly anymore to hide this obnoxious crap :(
Edit 2: That's hilarious, they seem to have borked the whole site at least for me with this change.
Giant recommended videos tilemap below the video replacing the fairly neat comments page which has now all been jammed in the sidebar where the recommended videos used to be.