My hypothesis: This is just another indication that stock market speculation prices spend most of the time almost completely decoupled from the value of the underlying assets. Eventually there is some triggering event, lets just call it a Lehmanbrovent, when some massively leveraged party can't pay off a creditor, and the effect cascades through the market bringing it back to a real valuation. We call this state a "catastrophe." The decoupled state we refer to as a "healthy market."

A good way to test it would be to see whether the accuracy of the twitter indicator varies based on types of market activities, such as: does it completely fail on crashes, but always predict run-ups?