There's a significant difference of opinion about whether Bob was truly ready to retire. After all, despite Les Moonves' words to the contrary, wasn't there speculation of yet another TPiR lawsuit on the way, and that as a result CBS was "encouraging" Bob to just retire?
Some people say Bob had become stale and less than spry in his old age, to which there's some degree of truth. But I think he could've gone at least one more year.
Then there's Bob's statement that he'd rather retire too soon than too late, and then there's the school of thought that doing it that way always leaves people open to seeing you more (in the same way that a cliffhanger ending always brings audiences back).
Either way, his announcement DID seem to come out of nowhere, mid-season. There had been talk of such a thing for several seasons IIRC but there was no indication that things had taken on a more serious air come Season 35.