What would cause a patient to bite down hard during intubation?

Not yet ready: The patient is usually asleep before attempt of intubation. Sometimes however the medication has not been given time to take effect fully ( every pt reacts differently). But this can be quickly addressed by the anesthesia provider.

That would: probably be an emergency forced intubation. An intubation for general anesthesia will involve a muscle relaxant and a nerve stimulator after a dose of Propofol that that is effective immediately