It appears to be cracked, which I would expect under these conditions.
Since the periphery of the disc is moving master than the ID more work is being accomplished and more heat will be generated.
Since the outer pad appears to have a significant amount of friction material remaining, and yet the piston side pad is obliterated, I would speculate the the sliding pins have seized causing non-uniform wear of the pads.
You can see near the hub where there is a raised burr on the outer side, which indicates that the rotor was divided from the inside of the car to the outside. Most likely by the backer, or the piston itself.
:thumbup
curiously the rotor on the opposite side looks decent…
:crazy