All I can figure is that when the catch dogs got there he bolted and ran her over. There was a grunt, a break, a squawling dog, and then another bay further away.
We've had a dog killed, two dogs gutted, one punctured lung, and numerous pokes and cuts over the last 3 months and in every single situtation it went down just as you described. The cover is just too thick for the dogs to get out of the way when the hogs breaks to them.
Real sorry to hear about Annie and rest of the pack. :'( I completely believe in the concept that some hogs are true dog killers. How they get that way I don't know, but there are some out there that know how to carve a dog up too good for it to be random chance. Did this hog show any signs of being dogged before?