Yeah, I haven't been able to figure out if it's really a bug or not, was hoping someone In The Know would comment.
Judging from the cutscene, I was thinking that maybe the DK had become my "main char" for Act II, and thus the swap...but in the quest log, it's still from the point-of-view of my other char (not the DK). Weird.
The thing that swayed me towards thinking it was a bug was that in the list of fixes for one of the patches, there's a mention of chars being swapped, and it being fixed...