The threat of arbitration
Keep in mind that management hints of calling off the merger could be a pressure tactic. They might proceed with a merger anyway, leaving the list to be settled later (probably by an arbitrator), which is the usual outcome. The possibility of imposed arbitration can be helpful at first, as each side hopes to reach a semi-acceptable deal rather than risk a horrible result from the arbitrator. But as arbitration becomes very probable, they dig in their heels, lest a generous "last offer" become a starting point for "splitting the difference". In addition, negotiators would rather have a bad settlement imposed than be on record as agreeing to one -- the arbitrator doesn't have to return to the cockpit and fly with angry colleagues.