Maybe damage was due to bud 9 dwarfing rootstock?
I'm in a similar zone nearby and my young apples are doing fine.

It does sound like freeze damage. There is something called southwest injury which happens in strong sunny weather in the winter, maybe you had the perfect conditions for that. If the broken bark is on the southwest side that would add more evidence (it is what gets the hottest sun). I have never seen it and the fact that it killed the trees back to the grafts is very puzzling, usually it just damages the bark. One other diagnosis is if the bark cracks are blackened it could be a low fireblight strike.

Scott

Are your trees in a low spot....I had allot die on me until I learned to
plant them on higher ground.

Thank you gentlemen. As for southwest injury, there are plenty of shady trees all around the damaged trees making that unlikely. As for a low spot, my whole yard is a low spot. On one tree the rootstock is growing out, so I know that is not dead, just the much desired scion died. Is it normal or possible for a graft to fail after healthy growth for a full season or two? Thanks. Ted

A picture would be a little more helpful.
Sometimes it's hard to tell why the graft failed after a season or two.
Even for zone 6 you could have hardening off issue in a low spot, especially when your fall period is wet and wood can't harden off properly.