The Mortgage Bankers Association reported a drop in mortgage applications and refinance applications last week. In this video, Motley Fool financial analyst Matt Koppenheffer tells us how, if this is the beginning of a trend, it could be very scary for the banks right now, as the fees from these mortgage and refinancing applications had been a major part of banks offsetting their shrinking net interest margins. He also tells us why, if this is a trend, it could be particularly bad for Bank of America (BAC 0.46%), though he warns that we shouldn't jump to conclusions yet over one week's worth of data.