PTWM,
First of all it looks as though you have one heck of a nice huntin spot. You're very fortunate and I'm a little jealous.

As far as the dead deer go, by looking at the pics it appears to me that they were not all killed at the same time. The reason I say this is because the carcasses/bones are all in different stages decay. Some of the bones look older with green moss growing on them and no hair around them while in the last pic you can see hair all around the body including still attached to the flesh near the hoof.

Is it possible that this could be a neighbor or poacher dumping site?

If not, maybe an EHM outbreak?

It would take one huge pack of coyotes to take down 5 deer all at the same time in the same place.

Either way its a shame that happened, especially in your honey hole!