I don't know. Adams was just SOOOO bad here, I have to wonder if maybe we got fool's gold and he was never as good as he looked in New York for some reason. Perhaps they had a system that really kept his deficiencies hidden, although it wouldn't be like the Jets to run their organization that competently. He probably just got paid and stopped caring.
Its actually pretty simple. He came to a team that forced players into a system with little to no flexibility within it to take advantage of their unique strengths.
When he first arrived, Pete, without time to have Jamal completely learn the safety position in his offense allowed Jamal to be the Jamal we saw in NY - a quasi LB / 'in the box' Safety hybrid that did little in coverage. His first season here, he excelled. Thereafter, Pete was intent on forcing him to play the system outside the box - something Jamal should have never been forced to do to the degree Pete tried to.
Jamal (ironically, like the qb eho played across the ball from him - Russ) was an elite talent with off the chart ability in a few areas, who required a system to be uniquely tailored to his skill to get the best from him. Build the system to his abilities and you have something special. Expect him to play the game in a traditional way and, well, you get what you get.
Who knows what might have been had he been put in a better position here to excel. More likely than not, the injuries happen anyway and no matter how he'd been used, he would have been a ticking time bomb. But at least we would have gotten more from him (and him, more from his career) while he was here.