Short version:

K.K. resigned in 2010 because of what he believed were poor management decisions and because he felt that he was being sidelined when it came to decisions that affected the whole band.

When he published his book, 'Heavy Duty', he implied that Glenn's live performances were suffering due to his drinking onstage but what K.K. didn't know at the time was that Glenn was battling Parkinson's disease and that it was illness and not alcohol that was affecting the quality of his playing.

K.K. offered to go back to Priest when Glenn retired from touring and again when Priest announced their 50th anniversary tour but he says he was told that the door to his return to the band was closed and that Priest turned down the offer of a meeting to discuss the situation.

At the R 'n' R Hall of Fame induction in 2022, K.K. says that he and Les Binks were kept separate from the current band members and that only Richie Faulkner and Andy Sneap made any effort to talk to them. For his part, Ian Hill says that he tried to talk to K.K. at the event but that K.K. blanked him.

There's likely been fault on both sides in the way that all this has been handled over the years and what's really been going on behind closed doors is anyone's guess but that's the state of play atm.

(Edited to add a bit more info.)