The higher the co2 pressure the more quickly it is absorbed, in addition, shaking the keg increases the surface area of the liquid presented to co2 pressure which also allows it to be absorbed quicker. It sounds like with the combination of the very high pressure and shaking you have over done it a bit.
You can de-carbonate by periodicaly releasing the pressure, this may take a while though.
Next time set your desired pressure and shake, you should hear the co2 rushing in as it is absorbed, when it becomes increasingly difficult to get any more co2 to dissolve, stop and allow the process to continue un assisted.
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