When dead skin cells on the scalp sheds faster than it is supposed to resulting in noticeable flaky skin on one's hair. It is usually the result of a greasy scalp which is not always your fault because some people produce more sebum than others which may be the result of hormonal imbalances. There is a common misconception where it is caused by a dry scalp, but it is caused by the polar opposite.
I've had greasy hair with dandruff before and it is not fun. Take care of your scalp peeps!