Part of me thinks this could be the reality. First 3 years Kamara was with Drew Brees and competing for a super bowl. Last 3 years has been with a QB carousel and failing to even make playoffs.
A 2nd year with the same QB brings stability and the addition of Kubiak brings an offense that should work better for him and get him in the right situations to succeed.
We all know players make business decisions about how they play. I think its possible Kamara held back in recent seasons because laying it all on the line for a team that clearly isn't competing for a title might not be worth it.