It was invented immediately after I visited Dan for spring break last March and I got to hear a lot of the process of inventing it.
Dan was still thinking in terms of TAPOS and faced the challenge of selecting block producers for short-term security (the chain is ultimately secured by stake-vote via transactions approving what they think the state is). There was a plan to develop a ripple-style consensus algorithm called Unity, and we considered just using a Trustee (timestamper) for TAPOS. The problem with both is that you can't easily replace the master nodes.
The key insight is that CDD applied to a particular block is the same as that piece of stake voting on each block from that chain as it sees it. If your stake can be delegated to a single block signer then old-fashioned DPOS provides the same security as TAPOS with 1% stake voter turnover per block. I think with approval voting that DPOS no longer has the same security properties as TAPOS in terms of stake-vote influence anymore, I think it's possible it could be strengthened by layering TAPOS on again.