When there is a sudden drop that large it generally means someone unloaded an immense amount of shares so it dropped the price of shares. Someone wanted to cash out and move their money onto something else or for whatever reason they may have. It doesnt meant that the company is doing bad or something happened, it's just a large 'sell' trade that was executed. Just as if someone were to purchase a large amount of shares (larger than large I should say), then the price of shares will generally increase a good amount from one second to the next. I believe SIRI will go back up tomorrow, I wish I had extra funds laying around in my account because I would have bought more today when it dropped in the $1.50 range.
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