If lab partners form a hypthothesis together and write the same hypothesis worded differently, is that considered plagiarism? My bio lab TA seems to think that lab partners cannot have the same hypotheses. My partners and I formed the hypotheses together, then we individually wrote them down our own ways. Now she's claiming we plagiarized, but I don't see how that could be considered plagiarism?

For reference, I put "There will be no change in population density between days 7 and 14." My lab partner put "There will be absolutely no change in the density between days 7 and 14."
Plagiarism is copying someone else's work and representing it as your own. This is not what you did. The hypothesis is joint work, even if each of you wrote it down individually. The differences in wording are not relevant, it remains joint work.

If any of you would present the hypothesis as their and only their work, this would be misrepresentation of authorship, which is also an ethical misconduct in research. On an ethical "seriousness scale", I would consider it to be about as bad as plagiarism.

To do things correctly, you should note down all contributors when writing down the hypothesis. Using slightly different wording that does not affect the meaning does not change anything, although to avoid confusion I would actually recommend to use the exactly same wording.