Officially: no reason. And I get this; I vaguely know how CA employment law works and that you limit your liability by not stating a reason. It's also really hard to work through in your mind.

The best theory I have is that, two weeks earlier, I raised concerns about donating 10% of ad revenue to charity. Some management likes getting feedback, some doesn't.

The reason I had concerns was that this was revenue, not income. That means you need ~10% margins to break even. This can be hard to do; Yahoo and Twitter don't. Salesforce does something similar, but it's more all-around, and in a way that promotes the product without risking the company's financials.