The first is the simplest--it's the economy. While online advertising is supposed to be sheltered from the brunt of the recession, advertisers may be pulling
back from a more risky Web option like a content network buy, in favor of a safer, pure search pick. Other reasons stem from changes Google has made, from reducing the amount of clickable ad space
(thereby reducing accidental clicks), to possibly cutting better profit-sharing deals with larger publishers (and leaving small publishers holding the bag).
Jason Lee Miller notes that all these assumptions come with the expectation that the publisher has followed the rules (i.e. great content, good SEO and plain business common sense). The bottom line is that site owners will need to get used to the fluctuations, and make a concerted effort to diversify beyond AdSense.