Interesting. Motor requires fuel, air and combustion to run. So for fuel: surprised it was pulling enough fuel to run that high. That said, I have seen a diesel 'run away'. It takes a while to run a float chamber out of fuel which is probably why it was running for so long. I have in times past shut off the fuel and let my K5 run the fuel out of the float bowl. It is a BAD practice and takes a while for it to run out. I do not do it any more.
On the combustion, either something was 'glowing' in the cylinder or it was dieseling from really high compression. With self supporting combustion you are kind of screwed until it runs out of fuel, or you have to cut off the air to get it to shut down, or lower the compression to the point where it will not support combustion, as you did.
Have any air leaks in the engine? That could easily explain the RPM raising with the throttle closed. It would also get things hot in a hurry in the combustion chamber.
I am sure it was a hair raising experience!