Google terminates advertising partnerships regularly based on fraud or consumer protection issues, or just because. They never comment publicly on any particular instance, but it’s usually pretty easy to guess.
The most recent example is EcoCho a new search engine that says they donate part of revenue to carbon offsets. I made fun of them last week when they launched, since the exact connection between their revenue and the carbon offsets was rather vague (they say “up to two trees” will be sponsored for every 1,000 searches on the site, which has exactly no meaning whatsoever).
Google has terminated them, the company says. That leaves EcoCho with only Yahoo to provide search and advertising to the site. You may want to go try out the service before it shuts down, because this thing is doing a belly flop into the deadpool.