Since rancid oil produces free radicals, would that rancid oil if mixed with fresh oil cause the fresh oil to go rancid as well? 
As you cooked by batches the last one add brought much more water than usual because   it was the last portion of the pack and usually ice pieces are coming down as well the frozen balls kept condensing water on them for all time you fried the first batches.    Except if meantime you have discovered a malfunction of the fryer so that the T goes so high at the heating element to cause the oil to start fuming.  