Lam and Sandy families of Rockingham Co. VA

Looking for ancestors of William Harvey Lam (b. 1837) and Elizabeth Catherine Sandy Lam (b. 1837) in Rochingham Co., Va and moved to Nampa ID between 1883 and 1903 with 8 of their 10 children, some of which were already grown with spouses of their own. My question - why did they all up and move clear across the country?