The crack was completly invisible until the weight of the horse forced the Coppertox through the hairline cracks.
OK. You poured the coppertox on his soles? So there is a lot of separation that allowed the coppertox to go right through? If not, please explain. What did the farrier say / do about the separation? How did the farrier trim when the crack was discovered? What else was done, by you, farrier or vet? Have you had a good vet out to him in the last year? If so, what have they said? If not, I would do so. Hoof pics would help to get more specific idea/info from us mob.
My common sense tells me you would not want any traction on that foot, like a shoe... Now that the hoof has grown out completly we put heart bar shoes on him but he is still lame until he gets warmed up.
Why have you shod him? Why with heartbars? Lameness that goes away/lessens with more exercise may not be related to his foot.
I would just like to know what could cause this. Is it stress from all the twisting of the foot from pacing the fence?
No, I don't think that would do it, at least not without an underlying problem of which the pacing just exacerbated. Sounds like a massive abscess that's blown out the whole back of the foot. Could have been due to an injury, as Trinity suggested or due to very unhealthy feet, untreated infection, etc, of which a minor trauma, environment, etc may have been the trigger. I've only seen this severe an abscess once, where the whole hoof blew out at the coronet - I think from a laminitic 'attack', but thankfully the wall below was firmly attached & without infection, so it was mainly a matter of trimming to keep the wall unloaded until that area grew out. Also seen a couple of massive subsolar abscesses which have effectively blown the whole sole away, down to the corium.