si.com/nfl/seahawks/news/report-carlos-dunlap-could-return-to-seahawks-after-testing-free-agent-market
"...However, after being re-signed last March following a brief foray into free agency, Dunlap struggled during the first 11 games and found himself in an increasingly diminished role for Seattle's defense. He produced just 0.5 sacks and four quarterback hits through Week 12, losing playing time to the likes of Rasheem Green and Kerry Hyder. In Week 12 and Week 13, he played fewer than 10 total snaps.
Dunlap did find his groove down the stretch, racking up 8.0 sacks in the final six games, including a trio of sacks in a loss to the Rams in Week 15. With that pass rushing flurry, many expected the Seahawks to keep him on the roster for the second year of his contract, but that ended up not being the case with the team transitioning towards a hybrid 3-4 defensive scheme..."
It could be anything, money, age, skill set, performance, attitude, conflict in the dressing room, whatever. The team didn't want to retain him at $5.1M...so they didn't. That he's still a FA suggests they made a correct decision.