I agree with Mom2Beagles.... this is odd.
I think she has a good point with her suggestion that the older dog may have been ill and aggressive.
Since you mention they are hunting dogs, I assume that they do not display people aggression.
The food guarding/aggression that you mention is not, as you know, uncommon with Beagles. There are things you can do, from a behavioral training perspective to address this but, I really doubt that is what caused the incident.
I also assume that it was not rabies or something like that because I would think that as hunting dogs, they are current on shots.
One suggestion that I would offer is that you may consider taking the other two dogs involved in the incident to a veterinarian to rule out any health issues.
I am really sorry that I don't have anything more constructive to offer.
Again I am sorry for your loss. I know that with 29 Beagles you too are a Beagle lover as am I. I bet they are a joy to watch on the trail!
God Bless You... Have a Happy Thanksgiving