Yahoo announced their quarterly earnings yesterday afternoon.

In the days leading up, analysts estimated that Yahoo's earnings would exceed their expectations (go figure out the paradoxical logic on that one), and sure enough, they did.

What's more, Yahoo upped its estimate of next quarter's earnings.

So why did a major analyst downgrade the stock this morning, sparking a 15% tumble?

To be more specific, why did they do it this morning? The reasons they gave were truer yesterday than they are today, and making any upgrade or downgrade immediately after an earnings release sets the tone for telling investors what they should think of that release. There goes a hard quarter's work...