It was a violation of arming and safing procedures by the munitions crew. They connected electrical plugs to the pylons of the F-4 before it was ready to launch at the catapult in violation of standing precedures and direct orders. This sent a spurious signal that fired the Zuni.
pfflier, it was not a violation of standing procedures and direct orders that resulted in that spurious signal.
It was a calculated risk, they knew it was a risk to have the cannon plugs connected at that stage of the launch cycle, but they were trying to speed up the tempo of flight operations at that point in time. (My understanding of it is that connecting them at that point as they were spotted on the fantail rather than waiting until the aircraft was on its way or actually on the catapult saved some amount of time they deemed worthy of the risk)
They discussed the risk at multiple levels and deemed it an acceptable one and got it approved as an additional method to accelerate the launch cycle.
Unfortunately, it was the wrong choice and men died as a result.