I would think it has been cut maybe caught in some roofing iron or the like or it has been strangulated by something like twine wrapped around it then the dead part fell off.
It would take a lot of force to just pull a tail off in that position as it is very strong and a straight force like that would more likely cause it to separate up near the sacrum and causing some nerve damage affecting her gait and around the vulva.
Holy cow! I've had them ripped off but never that high up. Agree that it looks more like something was wrapped around it and the dead part fell off because there's no residual blood or other signs of trauma. The ones I've found were a veritable blood bath and in one case, actually found the rest of the tail wrapped around a tree.
Wolves came to mind, and so did fescue as well as possibly coyotes. But, and it's been already noted, this is too high up and looks to be a clean/fairly clean cut. Looks as if it were almost 'tied off'. And fescue would just be the switch.
I've seen some places that have old hotwire/barbwire laying around. (We just cleaned up a small spot I've got for the winter) I've seen cows get tangled and drag that crap everywhere too. I suppose it's possible she got it wrapped around her tail and maybe made a clean cut herself!
Especially if the other end of said wire was still attached to something sturdy.