Locals say it's all "Californians," but that is just short hand for folks out of state but surveys and data from ITD (Idaho Transportation Department, i.e. DMV) show it's urbanization at work.
Granted there are folks moving to Idaho from outside of the state but they are the minority causing the influx to CDA and Boise.
Source: Local news, resident of Boise, and someone who is dismayed at the lack of housing in the area.