We noticed that our backup server had quite a high load. Bacula told us that it was spooling to disk (hadn't yet touched the tape drive). I did notice two processes with high I/O weight, [migration/0] and [migration/1]. What are these? They are something in-kernel, methinks, because they have quite low PIDs (2 and 4). The server was just barely swapping, less than a MB.
Looks like they're the part of the scheduler that moves threads between processors. I'm still unsure as to how they cause load to spike so much, though.
I also had this migration/0 keeping 99.99% in iotop. It went down to 0 after I killed a stuck user process. It was a kind of trial and error method so can't give more details on it.