Why worms group up in a big ball

Just saw this and suddenly it all falls in to place.

BBC - Earth News - Earthworms form herds and make "group decisions"

I have often wondered why when you put your worms (of different types even) they all clump together, maybe this explains it! I thought they might be doing it to share eachothers moistness (errr) or even having a giant, erm, how to say this without being rude.... naked party!!

I always thought it was the rude meaning till now .
Thanks for that!!

why are they called a herd? why not a slime of worms whatever it is i dont care as long as fish keep eating them,who pays these people to do this "work"