I have noticed about two weeks ago that when my cat came back inside she was losing hair on the top of her head. There is a bit of dandruff, but the hair is slowly growing back. She is also showing signs of pregnancy, so if this is something to do with that, I have no clue.
An unhealthy diet, poor health or an undetected disease can all be causes for your cat to lose their hair. As these are all quite broad reasons, it's important to visit your vet to identify the underlying cause. Some causes for hair loss in cats can also affect humans, and are contagious.
According to this article at purina.co.uk.
Anxiety medications, synthetic feline facial pheromone spray, or other calming aids could prove helpful as well. Regular tick and flea control medication is also essential to keep the cat healthy and rule those pests out as a possible reason for hair loss.
According to this article at thesprucepets.com.