There could have been a problem the PCM detected,,maybe with the Rear Wheel Speed Sensor (sensor had an erronous value at one start up),, and turned your ABS/Brake lights on. Then after a few key cycles the problem was not detected anymore, and the PCM turned the lights off.
Also,, JR2 here on the Boards had a problem similar to this,, and it turned out to be that with high idle devices.
If a Dodge/Cummins is Idled up above 1,200 or 1,400 RPM for a set amount of time the ECM/PCM assumes an error with the Rear Wheel Speed sensor and sets the Brake/ABS Lights also,, when pulling codes it will say the RWSS is faulty,, after a few key cycles the light will turn off.
Hoe this helps,,
I hope I was clear enough also,, My grammer is not always up to spec.
Merrick Cummings Jr