Grain stress. This will also cup a board. Removing a lot of wood from 1 side. Glue up your wood in strips 3 to 4 inch flipping the grain is the best way to fix this. You can also glue Baltic Birch plywood to 1 side router the edge and fill the end grain. This is also used for thin carves 1/2 to 1/4 inch to strengthen for apliques.