@zeus, your other site in the same niche, is all of its' content in exactly the same niche (i.e. would you expect all pages on both sites to be competing for the same terms)?
Do both sites use the same content (or are they based on the same basic content)?
Does the non-affected site get much traffic?
How long has the example page you mentioned been ranking for that phrase? I often see pages rank in my niche suddenly start to rank really high when I don't think they deserve to, but they fade away after a few weeks.