why does dandruff happen?

I think it's a combination of an oily and dry scalp

Dandruff is really just dry skin on your scalp. The sensitive skin gets dry and then it peels and flakes off. The best thing you can do is use a moisturizing shampoo and conditioner to keep your scalp moist. Sometimes though there is nothing that you can do. The cold winter promotes dandruff and because it is not easy to just put lotion on your head it is not always easy to get rid of. Hope this answers your question.