Cupping and crowning are common complaints that develop with high humidity. Both problems occur across the width of the flooring material.
Cupping is when the edges of a board are higher than the center. It can occur after water spills onto the floor and is absorbed by the wood, but high humidity is more often the cause. If the wood expands significantly, compression can result as the boards are crushed together, deforming the boards at the edges.
Cupping is caused by a moisture imbalance throughout the thickness of the wood: the wood is wetter on the bottom of the board than on the top. The moisture imbalance can be proven by taking moisture readings at different pin depths.