There are three generalized categories in which amnesia could be acquired by a person.
The three categories are head trauma (example: head injuries), traumatic events (example: seeing something devastating to the mind), or physical deficiencies (example: atrophy of the hippocampus).
The majority of amnesia and related memory issues derive from the first two categories as these are more common and the third could be considered a sub category of the first.
Head trauma is a very broad range as it deals with any kind of injury or active action toward the brain which might cause amnesia.
Retrograde and Anterograde amnesia are more often seen from events like this, an exact example of a cause of the two would be electroshock therapy, which would cause both briefly for the receiving patient.
Traumatic events are more subjective. What is traumatic is dependent on what the person finds to be traumatic.
Regardless, a traumatic event is an event where something so distressing occurs that the mind chooses to forget rather than deal with the stress.
A common example of amnesia that is caused by traumatic events is dissociative amnesia, which occurs when the person forgets an event that has deeply disturbed them.
An example would be a person forgetting a fatal and graphic car accident involving their loved ones.
Physical deficiencies are different from head trauma as physical lean more toward passive physical issues.
The difference would be having surgery that removes part of your brain, this would be active and thus head trauma, while the surgery caused the surrounding areas to atrophy, which is passive.
Henry Molaison is a great example of physical deficiencies as parts of his brain began to atrophy after his surgery.
Amongst specific causes of amnesia are the following:
in which seizures are electrically induced in patients for therapeutic effect can have acute effects including both retrograde and anterograde amnesia.