Often caused by excessive temperatures and hard to spot, surface cracks and discolouration could be an indication to check the calipers.

Reason

Sticking brake calipers.
Brake/axel/wheel imbalance.
Disc blueing is also another indication of excessive temperature operation.
Pad restriction within the caliper housing.
Disc below manufacturers  recommended minimum thinckness.

Action

Renew pads.
Check/service brake calipers.
Renew discs if below minimum thinkness.