The truth could be a bit of both, writin possibly on the wall and an opportunity to move comes up.
In terms of not leaving a head coach job for it, John Mitchell did, England will be one of the highest payers of coaches and it is high profile, may well have got a pay rise, closer to "home", so whats not to like? Danny Wilson did similar with Scotland, Siebald was head coach of Broncos.
Hopefully it is a change which suits both parties.