While forging a wide chopping knife blade, this 1/4” 5160 had instant catastrophic failure right after coming out of the forge and before starting the next round of hammering. It still was orange hot, so I’m confused as to what happened. 

Was this new stock, or recycled (e.g., old leaf spring)?

Yikes! Was it leaf spring? could have had preexisting micro-fractures? 

I think you'll need to go through a rundown of what you were doing and using to do it. "Looks" like quench cracks. Need more info. 

...maybe too much difference in heat on the whole blade while forging, ....or too much cold forging which makes lots of tension which can result in such cracks.

in aspect that it is a wide blade I guess its both....but Daswulf is right, very little information

Brand new 5160 flat stock. Was forging the blade with conventional hand hammering technique and this cracking occurred all at once right after pulling out of the forge for the 5th round of hammering. There was no heavy cold forging, although there was some hammering after the color left to planish a bit. Is 5160 a steel that is especially in need of hammering very hot? 

If you hit it after losing its color you asked for failure.  You can only hit mild steels at black heat; not any of the high carbon steels or alloys

Yes....lesson learned....again!

The heat treaters guide app in the heat treating section gives you information on forging temperatures for various alloys, some have much smaller windows of opportunity than 5160. In fact, 5160 is quite forgiving in its forging temperatures, compared to some. 

What the previously mentioned app says about 5160:

Thanks Will.