Ken was reported as having been frustrated about the BBC's delay in agreeing his contract renewal. I don't know what the normal time-frames would be, but presumably it was reaching the normal time when you'd expect it to be discussed and agreed and nothing was happening.
I don't know whether Ken was pushing for it and being told to be patient, or whether he was waiting for the BBC to approach him about it and they didn't. Whether they were keeping their options open and/or trying to keep him on his toes by playing 'hard to get' - and maybe he didn't want to feel like he was reduced to begging them to keep him on - I really couldn't tell you.
Either way, the BBC claimed after the event that they had been planning on renewing his contract all along, but it seems that Ken hadn't been convinced of this - at least not with a reasonable notice period. Whatever happened, like anybody else, Ken would naturally want to have his job security planned and contracted in good time; it's not surprising that somebody in his position might have kept his powder dry and used his marketability and considerable negotiating power, and thus jumped ship when his confidence in the BBC and their motives was rapidly dwindling.
I think his age is also significant, as he would have known very well that the BBC have an in-built ageist agenda whereby they get frightened at the prospect of older, established people's experience and talent being valued and worship youth, or middle-age at most. They don't mind Melvyn Bragg or Joan Bakewell on the odd late-night high-brow programme that attract comparatively few viewers, and of course, ex-BBC programme controller David Attenborough is a sacred cow; but they just cannot stand somebody whom they consider 'old and past it' remaining very popular and at the top of their game in the mainstream.
He had also expressed annoyance that he was being pressured to play more modern songs, rather than the 'oldies' that he knew/believed his regular listeners wanted - so maybe the BBC just see younger employees as more pliable and less likely to question/resist decisions that they may see as very unwise?