Recall that through the OMT facility the ECB is willing to acquire unlimited amounts of sovereign  BIS central bankers’ speeches  3  debt, provided that the issuer has basically accepted the conditionality established by the European Union and it is acting in accordance. This was a master stroke by the ECB.
That changed on 9 August, after a large European bank announced that it was freezing withdrawals from three of its investment funds due to losses incurred by those funds and the difficulty in valuing their security holdings. This seemed to trigger a shift to a much more risk-averse approach by money market participants in dealing with each other.