Its a formal warning issued by central bank of any country which controls and regulates all other banks in that country.Whenever a new project or scheme comes related to economics,central banks issues a warning to people about it.Even the reserve bank of india issued a warning that bitcoin is very volatile and risky investment.But later,indian government changed its stand about bitcoin and had decided to legalize bitcoin.So,that's no need to worry about.But normal banks considering bitcoin as a rival is another issue.
I don't really thought that they do fear in crypto currency, even how popular bitcoin is but still it cannot give such great loss for the bank since crypto currency is not yet fully operated for all the transaction in the world. In the end many investors in crypto currency will still need bank transaction for the money transfer into real money.