B-frames. They are bi-directional frames, meaning they can reference data from either before or after them in order to assemble the image. The number ahead or behind they can go are reference frames, which in this case is 8. So, they are accessing either 8 frames ahead of them, or 8 frames behind them. 16 B-frames means that there can be a maximum of 16 of these in a row. My guess is it tried to assemble a frame from very little data, which requires a lot of CPU, resulting in the skip.
As to why the 1080p was uploaded, I don't know. I guess since it is free leech or something? It's not like it really hurts to have it up, as I don't see any other group doing this at 1080p Blu-Ray quality any time soon.