In Afroyim v. Rusk the Supreme Court ruled that US citizens may not be deprived of their citizenship voluntarily. A previous precedent (Perez v. Brownell) had made the possibility of possessing dual nationality very limited, and created a number of ways in which someone could be automatically and de-facto involuntarily expatriated.
What caused the Supreme Court's total change of heart? What was going on historically that influenced the court to change their mind? Perhaps some kind of theme in allowing multiple allegiances historically, etc due to international developments?