This is a process negotiation tactic that attempts to hamper the object of the dispute by pulling in unrelated issues as preconditions. This is a very common strategy among those who suffer from narcissistic personality disorder: if it is not a problem for them, then it is not a problem. It also can be a control or manipulation issue with SNS practitioners.
Examples include delegates from Vietnam refusing to meet for negotiations until they had received concessions on the shape of the negotiating tables, or one party in a custody battle saying, “I will not meet to discuss custody unless you pay for my hotel and airfare.”
To handle this tactic, discuss the process. If it is a small concession, do it. If a new precondition appears, walk. Reconvene later to give parties time to think. If this tactic is a stall or a signal of non-interest, it will be revealed in time. No negotiation will happen unless the parties want it to happen, and no negotiation will close before its time.