Why did the Bitminter application ruin my GPU?
Mining itself don't ruin the GPU, but the long usage. A GPU's lifespan is at least 3-5 years. When working continuously on high freq. it is decreased as well. 
  Whoop in your case, your card just got 'burned'
It is not a the mining itself that caused the damage, but there is something else, or software, or the GPU was not quite on her place, or maybe some malicious software helped you for the achievement.