I've seen detritus accumulate around the base of the standpipe inside an overflow. I imagine there was several inches of filth around the standpipe over many years, then the standpipe was pulled. The thousands of worms that were breaking down the detritus were all drawn together by the suction of the evacuating water and you end up with this despicable blob of worms.
This isn't a "bristle worm" problem, though. There was some negative comments about bristle worms in the video. The filth that it would take to sustain a population of worms like that is clearly the problem.
Yuck.
-Ed