His exit would have been very carefully coordinated, and he was probably holding on until agreement was made about his other ITV commitments and the wording from ITV and Holly in respect of his departure. Until he was satisfied of a 'warm exit', he was going to keep on broadcasting until they dared to kick him out or gave him what he wanted.
So I think the writing has been on the wall for at least a week or two, and perhaps much longer. Only now has ITV and Holly given in and agreed to appear sad about him leaving. I could be wrong, but I doubt he left without exit arrangements being decided between his lawyers and ITV/Holly so he can save a bit of face. That is probably what the last week has been about: his last bit of power over the team.