It's definitely not just about the money, as others have mentioned there's still $5.65m worth of dead money on his 2018 $7.3m cap number. His on field performance next season will be worth more than the $2m difference.
I'd say 51% of this is the slow off-season reporting cycle overreacting to the Hawks discussing potential trades, similar to the Sherman rumors that blew up last off-season. I'm sure many of our veterans have been mentioned in trade talks so far. Pete Carroll says "it's the time of year, conversations going in all directions" and it's viewed as confirmation.
The other 49% of this is probably the locker room/off field stuff. When Pete was asked about it late in the season he talked about how collectively most of the team was tired of it and wanted to focus on football instead. With all of the turnover we are doing they probably want to get younger and get more positive leadership in with a focus on the competition mantra.