In April, the average daily trading volume in agency MBS fell to $187.8 billion, the worst reading of the year and a possible harbinger of problems to come. One market participant, speaking under the condition his name not be used, likened it to MBS buyers “going on strike.” He added: “Right now, you have an illiquid market, and that’s not a good thing.” In December of last year, the MBS trading volume was...