Juan Brown currently has this pinned as a "highlighted comment" below his video. It was originally attributed to "Matt," whoever that is, and now is attributed to "Editor," although it's unclear who that is, too.
That comment has 440 replies, and the video has over half a million views.
As far as I can tell, it's a random comment, from a random person, and people see it pinned to a Juan Brown video and take it as gospel.
Aviation and Social Media are an interesting combination. Aviation is one of those things that generally inspire people. At the same time, the barrier to actually participating in aviation is high, both in money and time. Social media fills in the gap for a lot of people - you can look at YouTube and consume all kinds of aviation-related media (some of it of incredibly high quality).
At the same time, "in the land of the blind, the one-eyed man is king" holds true. Chuckleheads like Juan Brown and Dan Gryder race to post on YouTube with a video collecting the available footage and data from an accident. They present things in a digestible fashion for the general public. But then they jump off the deep end and add their wild-eyed editorializing. When they're wrong, there is no penalty. When they're right, they get to tout to the online masses about what great aviators and detectives they are. There is almost no downside for them.
They both found this new niche of "YouTube Aviation Accident Investigators," so I think a lot of their popularity is that they're first in the space. I can only hope better competition comes along.