It's because of sebaceous gland area enlarging that follows due to DHT (androgens) hitting on your androgen sensitive hair follicles. The sebaceous gland secretes the oil that is on your scalp, but due to the miniaturizing process it leads to increased secretion of the oil. That's why your scalp is so oily.
Here is a recent good study about that where they really looked at many men and performed a morphometric anlysis and they indeed found that in Androgenetic Alopecia patients the sebaceous gland area is really significantly larger than non-balding patients;
https://www.ncbi.nlm.nih.gov/pubmed/26147300
Got it; so Tsuji's solution will resolve both hair loss and oiliness in one shot!