The weird part of YouTube is typically an area full of video irrelevant to what you were originally watching. These videos would normally go untouched, but become popular when a significant amount of people follow of a chain of "related videos" into said area of YouTube. These videos usually have misleading titles and thumbnails, or are just plain... weird.
Since most people become confused when they somehow cross into the weird part of YouTube, they share the experience by commenting about it, usually to find that a great many of YouTubers have also reached this conclusion
"I was watching a video about Gary Coleman and 5 'related videos' later I got stuck in the weird part of Youtube."
"Somebody help me! I've been stuck in the weird part of Youtube for 5 days! I'm surrounded by deformed babies and old people!"
"Somehow I stumbled into the weird part of Youtube and I've never wanted to punch something so badly."