Capital continues to rush to the mortgage REIT sector as ZAIS Financial (RC 3.66%) just completed its initial public offering last week and Cerberus Mortgage Capital just filed for an IPO. ZAIS has been active since mid-2011, but Cerberus Mortgage doesn't plan to start operations until it completes its IPO. What does this rush of capital to the sector mean for investors? Well, for one thing, it suggests that conditions are particularly favorable right now or else companies wouldn't be rushing to go public. That should be a bit worrisome for investors in established mREITs such as Annaly (NLY 0.79%) and American Capital Agency (AGNC 0.87%) because it likewise suggests that the profitability and good times they've seen in recent years may not last.