Says DTC. =) You can inquire from your broker who can then submit the request on your behalf if the stock is DTC eligible and why they were put on trade for trade status. Normally the reason is not given in plain English, but they refer you to a 'code' which can then be looked up on the SEC website.
For instance, when I inquired about TEMN (which is on the trade for trade list), my broker inquired on my behalf to the DTC to obtain the reason and status. The DTC will then relay if the stock is DTC eligible and the 'reason' why it is trade for trade.
In the case of TEMN, it was the amount of price manipulation due to shorting. However, while trade for trade, DTC said TEMN has NOT lost its eligibility.
So you can keep your DTC eligibility and go trade for trade. And it is MUCH easier to get off the trade for trade then it is to get back onto DTC eligibility.
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