If you really want to know the "why" of what the media thinks are reasons for price movement, applied after the fact, sometimes at random, go to Yahoo Finance and enter the company name or stock symbol in the Quote Lookup box.

http://finance.yahoo.com/q?s=gm&ql=1

On the left sidebar, click on Headlines or Press Releases, etc.

Stock analysts with 30 years of experience don't know 'for sure' which way a stock will go. They can "speculate" on stock price movement based on company news, the auto industry, the overall economy, interest rates, raw material prices, labor costs, and a million other things.

Ever heard of "buy on bad news and sell on good"? It doesn't work every time. When it comes to the stock market, nothing works "every time"

GM has been correcting for more than a month.

But why did it drop so much at the opening of the market today (Friday)?

I know the whole market is down significantly. I looked in news items and can only figure that big China questions are behind the drop.