These levels are a product of what goes in, stays there. Some routes are: feeding, water added, additives, or other environmental things, and others I can't think of ...
As these critters need food, you are obviously feeding too much, hence the nitrate.
While it is true that these creatures play a major role in the symbiosis of the reef tank, it is assumed that a balanced ecosystem has an excess of nothing. When an elevated population of any creature appears, that is a system out of balance, and as the balance is restored naturally, as these worms will eventually multiply and eat themselves out of food, they will starve and die, thus leaving crews of decaying bristleworm carcasses, a system once again out of balance, to then be populated by some other organism that eats these, until extinguised, and on ad infinitum.
So there, yes, nuisance leves.
HTH
Tim