It's not being done in secret; everyone knows that the negotiations are happening. For example the U.S. government has a mini-website all about the TPP:

https://ustr.gov/tpp

The negotiations are being conducted solely by national representatives because the TPP is an agreement between nations. When the national representatives think they have a deal, they will present the proposed deal to their governments for ratification. In the case of the U.S. that will mean a Congressional vote.

Similarly:

Corporate deals are negotiated in private by company staff, and then presented to shareholders for ratification.

Domestic legislation is negotiated in private by Congressional staff, and then presented for consideration by the appropriate Congressional committee.

Etc.

National negotiators do seek input from their citizens who might be affected by the TPP. From the USTR site:

"On Wednesday, March 6, negotiators from the 11 Trans-Pacific Partnership Countries paused talks to meet with more than 300 global stakeholders at an engagement event hosted by the Government of Singapore. Representatives from academia, labor unions, the private sector, and non-governmental organizations from around the world spoke with and heard from negotiating teams about priorities for and progress on the pending trade agreement. The negotiators also had the opportunity to listen to 60 lecture-style stakeholder presentations."

https://ustr.gov/trade-agreements/free-trade-agreements/tran...

edit: speling

I guess there's some back story there. Perhaps in previous rounds they had problems with unfinished, known to be flawed drafts triggering big political upsets back home that ended negotiations early. Whereas if the process had been allowed to play out the final agreement presented for ratification would have actually been acceptable. I don't know though, that's just a guess.

That said, I try and assume the best in people. I really doubt there's some moustachioued cigar smoking trade negotiator explicitly demanding secrecy because otherwise the plebs might realise how much they're getting screwed. Seems more likely it's done for their own convenience. People always like to shield themselves from criticism by invoking secrecy, it's an easy "fix".

Free trade agreements are always negotiated in secret in order to enable the two sides to come to a comprehensive agreement. Transparent negotiations fail.