It's primarily an issue associated with TDA. That brokerage firm is not very fond of allowing trades on what they consider to be high-risk stocks.
But the DTC code is initiated by the SEC on a stock to keep it from being manipulated if they feel the price action stands the chance of further hurting investors. Type in "DTC chill" in Google and there's some pretty good answers about it. Here's a link in the meantime:
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