Relative pronouns (like quis/quis/quid) don't agree with their antecedents for case; instead, it gets that from its position in the subordinate clause. Note that they still agree in gender and number, since it's supposed to be a stand-in for the noun.
Because the pronoun stands in for a nominative use, the subject of the clause -- i.e. "servi non parent" -- it has to be nominative, which means it's "qui". Because "servos" is plural and masculine, the pronoun does as well.
You can find out more about relative pronouns here.