Well, I don’t know what an authority site means, but I would give you an example of what I meant:
You have a run-of-site text link on pokeranalysis.com. That link is among many other RoS text link. Wheather you paid for the link or not, it does not matter, but that website has over 15,000 pages. Getting so many links in a short time is not good. Links like this could temporarily hurt your ranking, but nothing serious in a long term.
As far as content, if you add content, without getting links back, it would lower your ranking.
But in your case, I think the most likely scenario is – you simply got out-linked. Other websites got more links since the last update than you did. And now they rank before you. I have been keeping my eye on a few networks which were banned a few years back, and now I see them coming up in the results. This means that Google did an update bigger than usual – news sites got in, newer sites got higher, older sites did not get enough to compete with the newer.
But it has nothing to do with an iframe hijack. I would personally invite anyone to hijack our websites with an iframe as I find it quite beneficial – the traffic and banners are still mine, and my logo is quite visible. To me iframe means free traffic, but that’s just me.