Help? I know my boneless chicken breast are done, specifically at 170 degrees, but why are they pink hued on the inside?

The brine could be one reason but also they need to rest for a few minutes once the cooking has stopped. Juices are still boiling inside and concentrating at the center. They will spread out as the meat cools down. Also be sure you are using a high quality probe such as a thermopen for accurate results. A tiny bit of pink isn't necessarily a bad thing, pure white meat can be very dry.