Lately, I've heard (and even seen) this happening with many OTC stocks. People "invest" in OTC stocks, get pissed if the stock goes down, and blame it on the broker. So some of the brokerage firms are trying to limit their liability by not allowing their clients to trade quite a few OTC stocks. E-trade doesn't seem to have the issue, though.
Also, if a brokerage is naked short on a stock, they will try to increase the likelihood of a stock going down by not allowing trades. It's criminal what happens, but it's the world in which we live.
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