John -- Hey there. It sounds like these couple are going down hill quickly. My best guess would be a combination of a couple of your suggestions. It looks like it could be sunburn--which perhaps affected the weakend plants more than the others. I recently had some seedling sitting side by side, out of the same flask in the same media and some burned while others didn't. Go figure. Maybe these few were more suceptible because of the mites and the shock of relocation.
Let me know how it goes.