The Zuni rocket fired due to an electrical short when the crew switched an F4 (McCain flew an A4) from external power to internal power. The ground crew had removed the safety pin from the missile prior to the power switch. Had they not done this, the missile would not have fired.
Here is the Navy accident investigation report:
http://www.jag.navy.mil/library/investigations/USS%20FORRESTAL%20FIRE%2012%20AUG%2069%20PT%201.pdfEdited by Spanky at 11:17:45 on 09/06/17