Since it happened so quickly and then stopped, that's a head scratcher. I know you would have caught any abnormal swings in temp or pH on your AquaController, so a temperature spike or a kalk overdose is fair to rule out. Anyway, glad you got it fixed, Ken.
When I've had problems with STN from the base up, it's always been because of phosphates. It isn't quick, though. The tissue slowly dies over days and weeks, rather than hours. As soon as I put a fresh batch of GFO in, the necrosis stops.
I think you are probably right that it's some sort of contaminant, which the carbon took care of.
Are things back to normal?