Nonbank mortgage lenders have been killing it in the GSE market in recent years, even gaining a dominant 52.3 percent share of new single-family business back in the second quarter of 2016. Recently, however, not so much. A new Inside The GSEs analysis reveals that nonbanks sold 45.0 percent of single-family loans securitized by Fannie Mae and Freddie Mac during the first quarter of this year. That was the third straight decline in nonbank share of the GSE market.While total Fannie/Freddie MBS issuance fell 27.1 percent from the fourth quarter, nonbank production was off 32.5 percent. And surprisingly, large banks picked up most of the slack.