When wood/bone char is heated to over 1000 deg F, they start to off gas carbon dioxide and carbon monoxide, this gaseous carbon is what gets infused into the steel at elevated temperatures.
These gasses also displace the air/oxygen within the container; carbon monoxide being slightly lighter than air, and carbon dioxide being slightly heavier than air. So you’re kinda covered both ways (some pushing up/some pushing down).
What you need to do when quenching is to ensure that the parts remain in this carbon envelope all the way to the water. Drop distance between the container and quench water is another variable, in my set-up I allow no more than ˝ inch.
Early on in my development I actually thought about adding dry ice to the quench tank, it would serve two purposes, cool the water, and provide a layer of carbon dioxide gas to the surface of the water, displacing the O2. That was a long time ago, never did try it… I may have to just for fun
So bottom line is that air is getting to your parts; it’s up to you to determine how/why.
Now if someone would offer me 10K, I would build them an almost foolproof CCH set-up 
