Cupping in hardwood flooring is caused by a moisture imbalance through the thickness of the wood: The wood is wetter on the bottom of the board than on the top. The moisture imbalance is  proven by taking moisture meter readings at different pin depths.

Causes of wood floor cupping:

1) The most common cause of cupping is a direct result of jobsite moisture conditions. This causes a difference in moisture content between the face and the back of the board, with related expansion of the back. The back or bottom of the flooring expands and is wider than the top; cupping is the result. The flooring affected by this condition will be tight together and without gaps.

2) Acclimation of the flooring to a higher moisture condition at the jobsite. When flooring is installed in an area with a moisture content higher than the flooring, the flooring acclimates to the higher moisture condition and expands. If no expansion gaps were provided, the expansion causes stress along the edges of the boards resulting in a slightly cupped floor.

3) Drying of the flooring: The face of the flooring loses moisture and shrinks, and is smaller than than the back. When wood loses moisture it shrinks, and shrinkage results in gaps. A cupped floor caused by shrinkage will have gaps.