Population growth is driven by two factors: net migration (people moving in minus people moving out) and natural increases (births minus deaths). Since 1960, most growth in Thurston County has been due to immigration, and natural increase has remained relatively stable.
Note that in the above chart from 1960-2016 there has been no years with a negative Natural Increase and only one year with a negative Net Migration (1969), where Net Migration dips below Natural Increase.
2016 was the first year since 2012 that the number of people moving into Thurston County year-on-year has decreased. Still, net migration in 2016 was more than three times the net migration in 2010.
While much of this immigration was likely due to the relatively stable economy in Thurston County, it is likely that the increasing cost of living in the Seattle metropolitan area also played a role as individuals from the metropolitan area looked for ways to reduce escalating costs of living - particularly in housing.