It depends on what the circumstances are. If communications are disrupted, nuclear forces on alert could mistake it for an attack and launch in retaliation. Daniel Ellsburg's new book details how launch authority, as a practical matter, was delegated to commanders in the field. If they thought that a war had started and they had no communications with their superiors, they would have had authority to launch on their own.

The problem is that one bomb going of could rapidly lead to 100's - who is to say it's an accident and not a strike? Wouldn't the enemy pretend it was an accident? Why are they moving forces now? (because they realise that a bomb just went off...)