The other way to view it is that they had multiple safety systems and they worked.
It should also be noted that even if the last switch had been made there would have been no explosion, as the High Voltage thermal battery in the weapon had not been activated and no electric current was available to discharge the x-ray unit in the warhead.
They reviewed the system after the accident and added additional systems to add further redundancy and eventually stopped airborne alerts.
See here, as usual copy and replace the asterisk with 'o'.
http://nuclearweaponsaccidents.bl*gspot.co.uk