From the pics that you posted I would say you have deer that died at different times/years. I know you said you were just through there recently in January, however the living and dying grass that's grown up around the spine in one picture and the mildew/moss growing on some other bones tell me they've been a while (like Sully's observations). Before I would go to any conspiracy theory of mass poisonings I would look at the likely hood of you having a case of EHD at some point with the possibility of a yote kill there on the skeleton with the rib cage still intact. I would venture to say though, that one likely was during the fall as there is no sign of hide even left, perhaps a hunter's kill that got dumped and perhaps drug to the bottom. How far away from water/stream would these be??