The Roberts situation may be fabricated, but I can understand if he did leave.
Bennett has been his rock, the one person who has complete faith in him. It is, was, worth staying here for less money, simply because of that stability, and the fact that, long-term, staying on the rails may end up potentially more lucrative than a big contract that gets torn up after a year or two.
With Bennett gone, and no scope to follow him to Souths (and even then, Bennett's time is limited), he's effectively back to square one. I know as Broncos fans we'd all say that our environment is the best for him, but in Roberts' eyes, it was all Bennett. And Seibold is an unknown quantity. The Titans would at least prevent him from uprooting his life, so if their offer is decent enough, it may be his best option.