Or they’re worried he will accidentally dislodge the mask and interfere with the proper administration of nitrogen. There is no actual danger to breathing nitrogen - the air we breathe is mostly nitrogen - we just need oxygen mixed in. It’s all a stage play for delaying his execution.
I’m not in favor of execution, but this is the most humane method we have available. So until we can stop executing people, it would be good not to fuck with doing it in this way.