Dandruff is the shedding of dead skin cells from the scalp (not to be confused with a dry scalp). Dandruff is sometimes caused by frequent exposure to extreme heat and cold. As it is normal for skin cells to die and flake off, a small amount of flaking is normal and common after detergent treatment.
Some people, however, either chronically or as a result of certain triggers, experience an unusually large amount of flaking, which can also be accompanied by redness and irritation. Most cases of dandruff can be easily treated with proper medication.