The main reason was that the drastic deterioration of the fiscal position and the health of the banking system in some peripheral countries in the Eurozone elevated to hazardous levels what has been called fragmentation risk, which in plain English means the risk of a breakdown of the European currency union as we know it.
This led to sudden stops in the financing of some sovereigns and their banking system, which in a way triggered massive capital outflows. All this in turn fed back into higher fragmentation risks, creating a very pervasive vicious cycle. The materialization of sudden stops in the Eurozone caught many by surprise.