Tailgating

When a broker places your order for a security, and then immediately places an order for his or her own account for the same security, the broker is tailgating.

Although this practice, which is also known as piggybacking, isn't illegal, it is considered unethical, because the assumption is that the broker is trying to profit from information the broker believes you have about the stock.

A broker will typically tailgate only when you buy stock in a sufficient quantity to potentially affect the price of the security.