p0603 KAM error question

[CassMori]

Today I had my CEL come on while pulling a hill loaded - this has happened before, in this truck and in my previous '02. CEL goes out after ~6 minutes. I had my Flashpaq handy today, and was finally able to read the code while the light was still on. Showed the p0603 KAM error, which from what I understand is supposed to occur when the batteries are disconnected, or similar power loss to the pcm.
Now that I have a working EGT gauge, I can see that this light/error code is reliably set at ~1275 degrees (yes, I know that's probably too hot - easy to hit with the flaspaq & a 12k 5th wheel). I was experimenting, at 1250, no light, 1275, light comes on, same code every time. Goes off at ~6 mins, unless 1275 is hit again while the light is on, then it takes ~15 mins to go off (6 mins from last time at 1275?). Temps don't have to be held at 1275, just touch it for a second, and the light comes on.
Previous to today, I never had a way to check the code before the light would go out (and no gauges to guess the cause), and it never set a stored code. Only happened once previous in this truck, maybe twice in my old truck.

Other than the standard reasons for this code, anyone have any idea what causes it? I initially was thinking it would be a backpressure/EGT too high code, until I plugged in and read it.