I know I've said this before, but honestly, half the time, YT seems to pick the worst possible points in the video to generate thumbnails for. I didn't mind so much in the old days when they automatically generated thumbnails from the quarter, halfway, and third quarter points of the videos, but their randomization process sometimes happens at poor points, such as scene transitions, motion blurs, empty settings, etc.
Admittedly, THIS default generated thumbnail was probably the greatest accident ever:
The MV actually relied on fading, so this was the only time generating a thumbnail during a fade worked out well... AND, considering they like to brighten up the default thumbnail for some reason, this just worked out really well.