Since he was the Chief Design Officer at Apple, responsible for all design, I imagine his role came with a lot of management overhead. I'd wager a guess that maybe he wanted to ditch the corporate overhead (and spend more time directly working on design), and as a bonus he'll get to take on a wider range of projects. Also possible that he wanted to go back to the UK and that his firm will be located there.

From Daring Fireball speculation his move to Chief Design Officer was already a move for him to be distanced from day to day design. Sort of a soft firing already, but by moving him up.

I think he has been independently wealthy for a very long time. Money is probably not the issue.

Yeah you have to be unhealthily greedy if money is still a big motivator at that level.

Or maybe he just got bored. It's been 20 years.

I know, actually 27 years, so from age ~25 to 52. Pretty much his whole adult life.

https://en.m.wikipedia.org/wiki/Jony_Ive

I think it's more likely that he wants to be able to have more input on projects outside of Apple and Apple aren't happy with design consultancy being part of their business. So the obvious resolution is to let him be independent, safe in the knowledge that Apple will always have enough cash to pay whatever it takes to get him to work on their projects.