A.Paul Fisette, a wood technologist at the University of Massachusetts at Amherst, responds: There is one thing that will make wood swell, and that is adsorption of moisture. By contrast, expansion of wood resulting from an increase in temperature is insignificant. Your boards were installed flat and are now buckling. That tells us that they have gained moisture since they were installed. The question is how.
You have ruled out roof leaks. One possible scenario is that warm interior air, which contains more moisture than cold outside air, is somehow leaking into the soffit cavities and condensing on the cold boards. The moisture content of the boards would rise, and they could swell as a result of increased relative humidity or the presence of water.
to read the full article.