This appears to be accentuated, and be more prevalent, in agricultural areas for instance, and may have to do with fertilizer by-products leaching into the water reservoirs where your raw water comes from. Not only can such type of water contain higher amounts of D.O.C., but often it contains higher amounts of PO 4 as well. I have listed only a few of the possibilities What one often finds is that several of these causes happen in the tank at the same time. The resulting amount of organic compounds that dissolve in the water is therefore high. Note also that this all happens as a result of normal metabolic processes that take place in the aquarium and that you, as a hobbyist are, actually, mostly unaware that they are, in fact, taking place.
Protein skimmers are excellent devices to do so.
The skimmer is actually undersized for the type of tank you have The animal load is higher than the protein skimmer can handle The skimmer is not operating at maximum efficiency. All the above are examples of a certain amount of organic material being removed, but the amount removed is smaller than the amount produced. The net result is that the amount of D.O.C. rises. Let it rise for long enough and undesirable algae will appear, guaranteed. Organic material is made up of protein.