They don't know
It wasn't explained to us because the vet didn't know
What else do you need to know?
There was no explanation to how he fractured the leg, it had been operated on for the repair of the cruciate!
Please see previous comments
They didn't know how it happened
You will have to prove that on the balance of probabilities (more than 50%) the vets were negligent in looking after the dog and this results negligence, the dogs of the injury. Seems unusual that whilst in the care of dogs like particularly when it’s just had an operation and probably kept in a cage. It remains the dog didn’t have a broken leg when it went in and it sustained one while in their care and therefore I think you should have little difficulty improving negligence.
If they insist on you paying the bill for the full amount and it’s not covered by insurance my suggestion would be to tell them that you’ve been advised that they are responsible for everything and they are responsible for everything resulting from the fracture including the fracture and that unless they agree to waive those costs, you will have no alternative but to pursue them to the extra costs which you have no alternative but to pay.
They should refer this to their insurance company so asked them for their insurance company details.
Can I clarify anything for you?