Aug. 2018,NY Long Island, 50 year old Pin Oak is dropping undeveloped acorn cupule or caps on deck,never happened before.
If your weather has been unusually hot or dry, this may lead to some acorn loss. The tree adjusts the number of acorns to match its energy reserves so that producing fruit/nuts doesn't exhaust the tree just before winter. Any change that stresses the plant will result in more immature drop than usual. Evidently, the red oak family which includes pin oak are rather famous for the number of acorns dropped early some years. Consider having the soil under the dripline tested to see if it is becoming depleted of nutrients. A soil test will give you corrective measures if deficiencies are uncovered.