The arrow could have ridden up a bit and caught the spinal cord where it comes down in to the body cavity. A buddy of mine caught it this year on a buck he shot and I was like, "wow, the spinal cord goes that low?" And sure enough, it does.
A thanks to Rack Attack for that bit of information as I didnt realize it at the time. Here is alink that shows how low the spinal cord is where it meets the body from the neck. The spinal cord is in blue.
http://home.mn.rr.com/deerfever/Anatomy.html
Anyhow, congrats on the deer. Like to see a pic.