Priest are supposed to kick off a retirement tour this year and call it quits, except maybe for one more album (wouldn't want to end their career with 'Nostradaums' now would they?). When this broke, immediate speculation was his health was bad.
After all, why skip a big payday? But he released a statement saying his health is fine, he was at increasing odds with the band management and decided he'd had enough. Weird, considering he's a co-founder and thus has an ownership stake in the band.
His replacement is Lauren Harris's guitarist, BTW.