Hissy is right, take him to the vet! Since it has'nt happened before, he could have an illness or disease. Here is a list of the most common medical conditions that cause aggression.
Whenever an aging cat shows a sudden change of attitude associated with aggression, hyperthyroidism must be high on the list of differential diagnoses. Affected cats show unexplained weight loss, hyperexcitability, increased appetite, increased thirst, and may have palpably enlarged thyroid glands. Gastrointestinal upsets and cardiovascular problems are also often associated with the problem. Confirmation of the diagnosis is by detection of elevated thyroid hormone levels (T4 will suffice).
This is a parasitic condition that primarily affects young to middle-aged outdoor cats. It is cased by aberrant migration of Cuterebra larvae. A sudden change in behavior featuring, sometimes featuring irrational aggression may be the only presenting sign, but other neurological signs, including sudden onset blindness and circling may also occur. There is no established treatment though the parasiticide, ivermectin, along with appropriate supportive therapy, may help.
The presence of a brain tumor should be considered when an older cat shows a gradual or sudden change in behavior, sometimes including increased aggression. Other neurological signs are usually also present, though these signs may be subtle. Confirmation of the diagnosis is by CT scan or MRI, and treatment, if feasible, is by surgical removal of the mass.
Sudden trauma to the head can cause a subdural hematoma or direct cortical damage with resultant changes in behavior. Surgical drainage of hematomas is sometimes possible and should be considered. Brain contusions (bruises) may resolve spontaneously or may be treated using corticosteroids or opioid antagonists.
Cats fed diets including large amounts of uncooked freshwater fish may develop thiamine deficiency because uncooked fish contains high levels of the enzyme, thiaminase. Signs of thiamine deficiency include an unkempt coat, hunched position, and neurological signs including altered reflexes, disturbances of balance, aggression, and possible seizures. Treatment is administration of thiamine by mouth or by injection and by rectifying the dietary problem.
Take him to the vet so these things can be ruled out. It could just be territorial aggression, but it's rarely against humans. Does your friend own a cat? Do you have any other cats?