With superconducters there's a phase transition if they rise above their operable temperatures. This can introduce sudden forces. In LHC for example loss of cooling has led to substantial physical damage to the magnets. I imagine a simple transmission line isn't quite so likely to be violent, but there's probably some bad things that can happen with this thing if it bonks.
Yes, there will be some mild violence underground if the cable fails. Probably more because of the liquid nitrogen than the electricity itself. Electricity stops almost instantly, as superconductors tend to be very bad conductors above their operational temperature.
If that happens, you pull the cable out, and fix the broken part (probably replacing some of the cable). You may need to patch some pavement too, if the cable is superficial.
