> It is a violation of your fiduciary duty to the client

It is a violation of being put in a position of confidence with respect to privileged information. Front-running clients is always bad. But one need not have a fiduciary relationship with a counterparty to front run them. Practically speaking, however, when non-client relationships involve front-running, we usually go after them as insider trading. (For example, suppose I trade for a hedge fund. I hear my PM is going to buy X. So I buy X. One, I front ran my own shop's order. But two, I also violated their confidence.)