As laid out in the legislation and the agreement, officers are limited. They can ask the person for their identity and their reasons for withdrawing.
Regarding the circumstances for the purposes of maintaining security of the border, an example might be if during the course of questioning a traveller were to provide false identity documents and upon the traveller's withdrawal the officer were to determine that the traveller was “border probing” in an attempt to find weaknesses that could be leveraged to carry out terrorist incidents. The biometrics captured prior to their request for withdrawal could be used in order to identify the traveller.
That's the reason for that type of wording—to maintain the security of the border.