I speculate his reason must be to design something massively popular, that so many people in the world touch every day, that blends computers and architecture. And that his reason must a project so big that it can't reasonably fit inside Apple Park, both in terms of physical space and in terms of manufacturing partnerships where the team must be on site.
I speculate he's leaving Apple to design the Apple car.
Replies: 1) If Ive wanted to retire, I'd expect he could; he has mountains of money, and has achieved tremendous success in the marketplace. 2) If Ive were forced out, I'd expect an announcement akin to "will serve as an advisor to CEO Tim Cook", not "founding an agency with Apple as a client". 3) Why leave now? Because Apple just bought a big piece of the car project, Drive.ai.