There are three main causes of ear bleeding.
1) Perforated tympanic membrane.
Also known as ruptured or burst eardrum. This can happen due to infection or change in air pressure. Middle ear infection can cause pus to form and stretch the drum. Eventually it may burst. The eardrum is like a thin piece of skin so may bleed if torn. Going up in an aeroplane or scuba diving can cause extreme pressure changes. This can also tear the drum.
2) Ear infection.
A severe ear infection can cause the skin to break down and ear bleeding to occur. The blood is likely to be mixed in with pus or watery liquid if this is the cause.
Damage to the skin from a stick or a fingernail can clearly cause bleeding. The drum itself is well defended at the end of the ear canal, although it can be damaged by an earbud or pointed stick. The rest of the outer ear is more vulnerable to damage. A bang on the head associated with blood from the ear can be a sign of a basal skull fracture. This is particularly the case if the bang wasn't in the region of the bleeding ear. Jaw damage or dislocation can also tear the ear canal and cause bleeding.
Anybody with blood coming from their ear or any other ear disorder must see a doctor.