John, did you happen to notice if their eyes looked infected? There is a problem with goldfinches, house finches and purple finches regarding conjuntivitis. I had one at my feeder (purple finch) that had it; one day he was just gone, but I assume he died.
If your goldfinches had this problem, then they say to wash your feeders weekly with a bleach solution because it is contagious to the other birds.
If they look like they've been injured, then it might be a starling or grackle that have juveniles in the area. They are very aggressive with other birds, even the cute little goldfinches. I've experienced that before and took away all other food except for the niger seed and finch seed (in tube feeders). I hope you find out what happened. Let us know if you do.