<human>: Could you distill the essence of the text into 1-3 sentences?

Hi, I'm a young and not-economics-educated teenager who doesn't understand this new crisis in the USA.
So this company decides that they should downgrade america's debt rating to AA+, knowing that this would cause everyone to sell stock, which in turn would cause it to drop (both NASDAQ and DOW dropped like 7% today) which would turn into a self-propogating problem. WHY couldn't they just leave our credit rating good, so that our stock market doesn't crash?

ALSO, why do economic expert people publicly predict a further downward spiral of the stock market, knowing this would only cause more people to sell, and create more fear of buying? Why can't they tell us, the market's looking good, buy buy buy!!!!! ? instead of exacerbating the problem?

sorry if this question sounds stupid, economics makes almost zero sense to a teenager like me. or maybe just me.
<bot>: Why did Standard and Poor's downgrade the US's credit rating if they knew that this would cause a steep drop in the stock market?