I can see how the mouth would close resulting in a bite either through reflex or simple mechanics since it is concievable that the head would not die until lack of oxygen stopped the brain and muscle functioning. (Ever see a chicken when it's head is cut off?) But lunging is the body pushing the head so there would have to be body attached or the chump put his hand in the mouth.
A doctor I worked with did a study showing that 90% of rattlesnake bite victims admitted to our ER fit the same pattern. The victim is male, between 12 and 25, if over 16 he has been drinking and the bite is in the hand. The remaining 10% do not involve trying to catch the bloody thing.