That was a cutoff from the AGC computer—it detected POGO oscillation in the feeds to the center engine and shut it down to save the rest of the engines, vehicle and crew.
I wonder if anyone really knows for sure.
My understanding was that although the inboard was oscillating at an incredible rate, it was a low thrust sensor that was triggered somehow and shut the engine down. Strangely, all indications were that the thrust level was adequate.
If this was truly the cause of the shutdown, either the sensor malfunctioned or the data was incorrect.
But I may not have the most current information either. But you might. Just the last I knew it was unclear exactly what happened.