Probably the one who was uploading to you ran out of pieces that you needed. (Partial File Sharing, also known as PFS).

Whenever you download from multiple sources, the best source is shown (but the others are waiting/hidden). If the best source has part of the file, but then runs out, you'll see your status change to the next best host.

It's like being in 20 or more queues at once. Some will give you a bit of a file (they may not have all of it), then you jump to the next best queue.