This page lists the major causes of scabs on cats. Scratching to relieve itching is the major cause of scabs on cats as the scab is formed to cover the self-inflected wound. You just have to figure out the reason for the itching.
Feline Scabies or Head Mange – This is a mite (a tiny parasite – Notoedes cati), the female of which tunnels into the cat’s skin on her head (and possibly the paws and genitalia). There they lay eggs. The eggs hatch and become adult and lay more eggs. The cycle takes 3-4 weeks. It is very contagious. The scabs occur because the cat scratches due to intense itching. Crusts are grey to yellow. Vets do a skin biopsy or skin scrapping.
Chiggers aka harvest mites or red bugs – this parasite is picked up in grassland. Likely to be in between toes and around ears and mouth. A magnifying glass reveals red/yellow/orange specks. They cause severe irritation. The cat scratches creating sores which form scabs.
Ringworm – this is a well-known condition (see pages featuring this disease). It is a fungus. Scabs and crusts can form leading to sores. Itching can cause scratching and scabs.
Ear mites (Otodectes cynotis) – another well-known condition. These are nasty parasites and one of the most common cat health problems. They cause intense itching leading to scratching which in turn causes scabs and loss of hair around the ears. Sometimes cats are also allergic to ear mites compounding the problem. The irritation is caused when the mite feeds, piercing the skin.
Skin diseases – one of which is mosquito bite hypersensitivity. This condition affects the tips of the ears, bridge of nose and itching of the foot pads. The signs are crusty sores with erosions and scabs. Cats who have this condition should be kept inside the home. The cat might have a fever.
Allergies – such as a food allergy causes itching, then scratching and ultimately scabs. Food allergies normally affect the head area.
Contact dermatitis – caused by coming into contact with a chemical to which the cat reactions badly. The chemical will often be in a household product such as a cleaning material or shampoo.
Feline acne – you’ll see blackheads under the chin. They can become inflammed and form scabs.
Kittens – poor sanitation of kittening box leading to bacterial infection – Newborn kittens can develop scabs, usually on the abdomen.
Allergies such as the flea bite allergy causes red blisters on the skin. I have not classified these as scabs. There is arguably an overlap here.