Did you ever somehow lose traffic to a competing site? Maybe it's a new site that just started and they did a massive marketing push to get started. I've lost traffic to other sites in the past. I've even seen some people lose traffic after a similar site did a somewhat smear campaign about a similar site, and it helped draw traffic from that site to theirs. I don't condone stuff like that, but just saying there are people who do bad things like that.