There are so many possibilities. It's frustrating to loose one and you just don't know why. Definitely looks like she's been dead since a few days at least.
Agree with what all the others said. Did you have lightning during those few days?
We had one cow with the legs back like that, still alive though. She had broke her back, well, bulls broke her back. But you can't check for that now.
Maybe she was in the process of having her calf, it got stuck and she died trying. Critters could have taken the calf and that's why she is open in the back. Or did she still have the calf in her when you found her?
Bloat, maybe. Also hard to tell because her behind is gone and the gas could have released.
And maybe none of the above and she just died of natural causes.
The behind missing......totally agree with what the others said, critters took it because it's the easiest way to 'gain entry'........... or was it Aliens? LOL Hey, don't laugh at me. There has been another 'assumed mutilation' last month in the area. Give me the answer to THAT. =)
So frustrating, I feel for ya.