At least based on what she has heard about it so far. From Vox:
In a major blow to President Obama's trade agenda, Democratic presidential frontrunner Hillary Clinton has come out against the Trans-Pacific Partnership. In an interview with PBS's Judy Woodruff, Clinton stressed that she hadn't seen the full agreement. However, she said, "As of today, I am not in favor of what I have learned about it. I don’t believe it’s going to meet the high bar I have set."
Clinton explained her skepticism by mentioning two of the most common objections to the deal among left-leaning critics: that it's too favorable to pharmaceutical companies, and that it doesn't include language prohibiting other countries from manipulating their currencies to gain a trade advantage.
Recall that she was for the TPP before she was against it.
If you listen to her full explanation, she does leave a tiny bit of wiggle room, but it would look pretty bad to reverse her position after everything she said.
If I were going to guess the reasons she would oppose TPP, those would not have been the ones. There was never any real chance of having binding obligations on currency; and most liberals seem to be celebrating the biologics outcome.
I'm not sure how much impact this will have on the Congressional TPP vote (I'd worry more about the Republicans), but it's certainly not what the Obama administration was hoping for.
