It could be because Paul called Titus his son. Titus 1:4 sais, "To Titus, Mine own son after the common faith.." I am sure there were unbelieving Jews who attempted to stifle Titus' job in Crete by saying that he was an illigitimate son of Paul. And according to the Law, this was a bad thing. It ws also an insult geared towards Paul. So I think this is why Paul told Titus to ignore such mindless banter.