As we can wood particles are not only one responsible for swelling
but also the condition
under
which they are presented into board.
Due to high pressure severs compressed crushing of particles occurs.
More
During exposure to water or vapour,compressed and crushed,cell wall will absorb water what will lead to regaining the original shape;shape before pressing.
I helped a friend translated a
paper.
This leads to release of compression stresses from pressing hence swelling occurs.