I had no idea there were so many potential causes of dandruff! The good news is that poor hygiene is not one of them. Dandruff can be caused by one or more of the following:
Skin is hydrated from the inside out. If you needed added incentive to drink more water, there you have it.
Zinc, B vitamins and certain fatty acids are needed to balance the skin.
Dry skin increases the chance of getting dandruff. This totally makes sense because I've also seen people with dry facial skin display flaking.
Apparently those with yeast sensitivity find a reduction in the amount of dandruff during summer months. The reasons for this are not clear: either UVA rays from the sun counteract with the yeast or a colder climate leads to drier skin there by making dandruff more likely in winter or even a mixture of the two.
Malassezia is a fungus though to cause dandruff. This fungus is present in on both healthy skin and skin free of dandruff but those with dandruff have 1.5 to 2 times the normal level.
Skin conditions, e.g. eczema, Seborrheic dermatitis and psoriasis cause dandruff.
Parkinson's disease causes dandruff but the reasons for this are not understood.
A reaction to a product
One of your hair products may have caused skin inflammation leading to itchiness and increased scaling.
Washing your hair too often can irritate the scalp and cause dandruff too.
Not combing your hair enough!
I too was surprised by this factor. The risk that your dandruff is caused by this is only slight. It occurs because combing (and brushing) help to clear shedding of skin. I wouldn't give too much weight to this cause.
Stress! - also appears to cause dandruff so chill out ;)
Ref. NHS, Medical News Today,