Isn't Tensorflow logo "TF"?
My second guess is that using "Open" together with something that implies "Terraform" is also a problem, as Hashicorp might sue them for defamation, as that name would imply that Terraform is not "Open". I'm not personally against that, I wouldn't consider Terraform "Open" anymore, but I'm not sure a court would see it the same way.
Given a product TerraForm, known also as TF, how likely is it that a consumer would be confused into thinking "OpenTF" is made by the same people who make TF?
That's the bar to clear. In the past, some trademark owners have been fairly lax, allowing unrelated "Open" versions of their software to be established. The classic case was "SUN OpenOffice", but I suspect they got away with it because "office" was not a wordmark - the trademark was for "Microsoft Office", which is very different. In a lot of cases, the "Open" versions emerged once the software was not sold anymore, so it was impossible to be confused.
In this case, though, the software is still sold and likely carries trademarks and wordmarks. Unless OpenTF's pockets are deep enough for some serious lawyerin', it will be easier to rebrand.
I'm well aware of that, I'm mentioning two potential issues, one being the "confusion" part and the other being the "defamation" part.