Well this one is interesting... from their announcement dated 18th January to investors:
Games Workshop announces that after five years of outstanding service Mark Wells is stepping down as CEO. He leaves on good terms and with our very best wishes.
The board will be seeking to appoint a new CEO in due course. In the meantime, our Chairman, Tom Kirby, will be Acting CEO and Chairman.
Faeit 212 appears to have an unsourced internal communication expanding on this a little, which in a nutshell is (and we're paraphrasing here) - the Chairman's job (Tom Kirby's) and the CEO's job (Mark Wells) were identical, so someone had to go for the good of the company (Mark). Originally brought on as the director of strategy in 2000, that makes it an (almost) 13 year run with GW.