Last week David H. Stevens announced he would step down as commissioner of the Federal Housing Administration, and this week its being announced that he will be named the next chief executive of the Mortgage Bankers Association (MBA). During his two-year stint in the Obama administration, he was a key adviser on housing-finance policies. He will succeed John A. Courson, a former mortgage banker who took the helm of the MBA in 2008. Mr. Courson’s departure is seen as a unexpected upheaval to a trade association that has had an extremely vociferous three year period.