Upgrade, downgrade scandal

Analysts at the world renowned, bankrupt and bailed-out investment banks still appear to hold significant power. They upgrade and downgrade stocks, set price targets and in general prescribe the future for the rest of the world. They must be either too smart for their own good (as we have seen enough evidence that the God's smartest creatures may be no good) or there must be something else going on here. How can a stock take a nose dive, after being "downgraded" by somebody, who use publicly ...