That is difficult to say for sure since there are quite a few possibilities. First I doubt that it was cold injury as your location doesn't get too cold for gardenias. It may have been due to soil conditions that were either too dry or too wet as poor drainage and drought both kill roots. When conditions are wet you can also get root rots that enter the plant and can lead to its demise.
If this happens in the future you can take a plant on its way downhill (not dead but dying) to your County Extension Office for them to either diagnose or send in to the state lab for analysis. Then a solution can be prescribed before it is too late for the remainder of the plants.
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