via Naturallycurly.com;

Every time you condition your hair, you leave behind a residue. It does not rinse away fully. Over time, this accumulation can be a problem for fine-haired people in particular. It can cause hair to look dull and flyaway, or limp and greasy. This is caused by cationic detergents in conditioners. If you try to wash away a cationic detergent (conditioner) with an anionic detergent (shampoo), a charge interaction occurs between the two, then you have build-up of both. Low pH shampoo deposits even more anionic surfactant, so this is not the solution. Short-chain surfactants: alkyl sulfate, alkyl sulfonates (sodium deceth sulfate) are better at removing this residue than standard sodium or ammonium lauryl/laureth sulfate shampoos. These are often regarded as being harsh on hair, but more effective at removing conditioner build-up without adding as much to the problem.