"Blue" brake discs on my GS

My GS developed a noise when breaking, and after some weeks the break performance deteriorated noticeably.
The mechanics tell me my break discs were "blue". With this they mean their surface became too hot at one point in time in the past. This caused damage to their surface, they had to redo it.
The problem appeared on all 4 discs, but they say was worse with the two discs on the rear axle.

We are trying to find out what caused this. I have neither spent any time on the race track (though this should not be a problem for a proper sports car like the GS) nor done any down-hill mountain racing. With all the past cars of mine, the breaks have never been a problem.
The only potential cause which might make some sense for me is that I have driven my GS on snow a little bit, and that the electronics have activated the breaks to stabilise the car (though, I have not done any special skidding, maneouvring, etc.). I have winter tires.
Any comment would be helpful, especially as (a) I am afraid without knowing the cause, the problem might reappear, and (b) the dealer is thinking about having me pay the disc treatment as the cause could be a maltreatment of the car by the driver...

By the way: the screeching noise when accelerating from stand-still and sometimes even shifting up, especially when driving uphill, seems to be caused by a spring in the clutch which has become too soft. It seems they need to replace the whole clutch...the problem started after 8k kilometers. The clutch wear after 10k kilometers by the way has only been around 10%.

Greetings
Wäschpi