Manifest destiny, the California gold rush and the signing of the guadalupe hidalgo are all things that encouraged mass migration West. The Gold rush had many people convinced they could strike it rich, but were then stuck in the west with no money. The treaty of Guadalupe Hidalgo expanded the US, meaning there was now more land to go around. Manifest Destiny is just that people believed it was "gods will" to move from the east coast to the west and expand.
I hope this helps.