From another board, but relevant here as well (by request of another poster here).
invenstor: EricVan - You are right. I have seen this before.
EricVan: guys..about the Broker restriction not allowing to buy..dont panic. It is normal for brokerages to restrict buying..the reason?! They are shorting the stock or have sold a big number of FTD (Failed to deliver stock). Hence, by not allowing retail to buy they get the chance to buyback first
I am not a broker. My statements are not to be taken as advice to buy or sell stocks.