Consider the following scenario. Experiments yield a somewhat peripheral finding. Not what I set out to find, but still interesting. When I encountered this finding, I looked up some past work, and found that it was corroborated in past studies.
Now: do I mention these past studies in the background, or in the discussion? Logically, it seems to make most sense to bring it up in the discussion. "I found this, which suggests this. And this is supported by a review of the literature". However, this might be against the "rules".
Meanwhile, bringing it up in the background seems strange, out of place, and like "leading the witness". This wasn't what I set out to discover, so it seems weird to discuss it.