From our state DNR biologist:
“This is most likely caused by a brain abscess which based on research over the last 20 years is pretty common in bucks, particularly mature bucks. It is caused by a relatively common skin bacteria of deer. Due to the tendency of bucks to rub, spar, and fight we all know that they tend to get beat up on their foreheads. In the right situation this common bacteria can take advantage of the sores/scabs to erode a small pinhole through the skull and set up an abscess in the brain. This will eventually lead to the deer’s death but it can take a while in many cases. If the abscess is near/on the antler pedicle, which is the bone from which the antler grows, it could cause this hollowing. I suspect that these deer have an enlarged, spike like pedicle that fits into the hollow in the antler.
The typical call that I get that turns out to be a brain abscess is from a landowner or hunter who says they have a nice buck walking around in circles in a field or food plot and you can walk right up to him. At this stage the abscess presents a neurological case and these deer are on short time. Brain abscesses are individual animal issues and do not represent a threat to other deer in the area.
Pretty interesting that you found more than one of these.”