This didn't happen because of any one or two recent events. This happened because this is what always happens.
Google got in on the ground floor of two evolving industries - search and online ads. Part of their success was due to their innovation, for sure. But there was also a dire lack of competition, and the fact that advertisers had nothing to measure them against. Could another ad broker outperform Adsense? Who knew? None of them ever lasted long enough for us to tell.
But now there's Facebook, other social media, Bing, and all those small companies that sprung up with an interesting twist on online advertising only to get crushed by Google's weight. After 14 years, it forms a picture of what else is possible, outside and beyond Adsense.
Advertisers aren't as willing to spend on Adsense now because they're beginning to question if they can't do better some other way.
This is all a very natural evolution, and it's been bound to happen from day one for Google. It may feel like karma coming around, but the truth is no matter how Google had behaved, eventually advertisers were going to wonder if the grass wasn't greener somewhere else.
Google isn't screwed. But Google's total dominance may be, and I hope it is - not because I want to see them hurt, but because it's just not good for competition. Google could still thrive in a more competitive atmosphere, and fine by me if they do. I just want the more competitive atmosphere, and I think that's where this is leading, and that's what I'm happy about.