Although, even in the academy officers are taught to explain the reason for the stop upon contact with the driver. While the LAW does not require this, today it is considered an element of so-called best practices to do so.
Also, the citation may not include the reason for the stop, so the cite may not explain why you were stopped. It is possible that an officer could have observed you roll through a stop sign and then, upon contact, found that your registration was suspended for some reason. He might issue you a ticket for the registration but not the stop sign offense. His notes on the back of the cite might indicate that reason, but the copy of the cite you receive would only have the registration offense and not the stop sign violation.