They won't breach the crewmember's privacy, but they've said the medical issue was not related to any mission operations. There had been some speculation online about it being related to spacewalk prep breathing protocols, but they have confirmed otherwise.

The crewmember is stable and they're not calling it an emergency evacuation. They could potentially evacuate in hours, but they're going to return "in the coming days" instead. They said it's a precautionary measure; they don't have the equipment onboard to do all the checks the doctors want to do.

Apparently they've done statistical models in the past that suggested the ISS ought to have had a precautionary medical evacuation every three years, so the fact that it's taken over 25 years for a single one is pretty incredible. (The NPR article mentions that it's a first in 65 years of NASA spaceflights, but that seems silly to say. Before the ISS, NASA had very few missions long enough for a precautionary medevac to even make sense.)