First, usually Wall Street firms do not issue very many SELL ratings for companies. It is well known that HOLD is secretly equivalent to SELL. Why, analysts believe management will cut off their access to them if they issue a SELL.
This is a good sign that the market is over extended. Sales or earnings miss and the stock gets really, really pounded! Furthermore, it spreads throughout the industry.
It's getting to that time where the sailing isn't so easy anymore and you need to keep your wits about you.