Correction, he contributed. he was a non-factor in NE this past year as well. I'm not sure why the complete dropoff in numbers happened this year, but it did start in NE.
Why was he traded again

I thought he had it good up there. I saw the coming back to Minn situation turn ugly coming because Childress is an ass.
My thought is there is a maturity issue with Moss and a get what you can attitude with Belichick.
Moss came into NE very motivated. Did all the right things...got his long contract, became a star again and then "got comfortable" in NE expected the "star treatment" which BB wouldn't stand for. NE saw a chance to get a 3rd round pick for a player that would be an unrestricted free agent in 12 games who wasn't giving it his all and made the trade.
Like with his banishment from Minn the first time and his time in purgatory known as Oakland, Moss got a wake up call, he wants back. Moss's first redemption tour in NE resulted in him breaking Rice's record, while I doubt he can do anything that amazing again. Moss being motivated to rehab his image could result in a very good player.