RE: What did this deer die from?
If you really want to know I would take a look at the brain and see what it looks like. If he was a big of a fighter as you think he was he may have had considerable brain damage from another buckIt can get pretty messy so you would have to be really curious.
That is a really unique situation, however. Let us know what you determine to be the culprit and if you get a buck on Saturday.