Analysts found prepayments surged notably in September.
Credit Suisse managing director Mahesh Swaminathan told this publication the latest numbers suggest, "a trend for a somewhat sharper response" in speeds.
However, that trend "is still in line to be weaker than last year's levels," he said.
Swaminathan said that there were particularly notable spikes in the speeds of Fannie Mae 4.5s and 4s that seemed to be "driven by a big surge in activity by a couple of large servicers."
Barclays Capital researchers, which also made note of faster-than-expected speeds in some Fannie coupons, said in a report they believe a pickup that occurred in 3.5s through 5s will be a one-time event.
They credited the acceleration in speeds to a 19 basis point rally in rates and two fewer collection days. They said there also likely were capacity issues in adjusting to a pickup in refinancing over the summer, resulting in a situation where some closings that normally would have occurred in August got pushed into September.
Analysts generally said they were expecting a rate-driven increase in refinancing but not as strong a response as actually occurred in September. Keefe, Bruyette & Woods researchers, for example, said in a report that prepayment rates were at the high end of their expected range.
