From what I have read the connection was at very low temperature to make use of superconductivity. The arc heated the connection and brought the material in the joint above the point of superconductivity.

Due to current flowing through the 7500 amperes heated the connection and surrounds before the backup system could dump the charge. So and there was a rapid rise in temperature. I do not know the voltage but for the electric trains in Perth it is 6000 amperes at 25000 volts. Each magnet is supplied an enormous amount of power to generate the field to control the beam line.

Magnets have exploded at other particle accelerators in initialization runs so it is not a new phenomena. What is new is the sheer scale of how much more powerful the CERN particle accelerator will eventually be.

As a note of interest in the universe today column some new unexplained physics with muons outside the beam line were discovered some months back by the Fermilab scientists at TeVatron link Physorg article.