for your whole life...you have combed or brushed your hair. Brushing massages your scalp, loosening dead skin. Brushing takes that dead skin from the roots and distributes it through the length and out the bottom. Your scalp has gotten used to that routine. For the past 4 months you have not brushed or combed. Your scalp is rebelling. It is normal. You probably have about the same amount of flaking, it is just more noticable because it is hanging around your roots now. Most people with dreads have expeirenced the same problem. Also the change of season effects our skin. Winter dries us out.
I found that by gentley scratching my scalp before i wash helps loosen up the dead dry skin. It makes it easier to wash away. Gentle is the key word here. you don't want a bloody scalp (It also promotes blood circulation which can help it grow faster). Either way....it feels really good