The video notably popped up in June 2018. At that time it was shared on Reddit and by Viral Hog, a company sells viral videos on behalf of the poster. Reddit doesn't offer much in the way of an explanation, but according to Viral Hog, the video dates back to April 2018 in Taranaki, New Zealand.
The caption to the video specifies that these are longfin eels in "fresh water flowing from Mount Taranaki." So, yeah, that's just a naturally occurring terror of eels in the wild. (Just kidding, that's not the name of a group of eels. It's actually called...[searches Google]...a bed?)