The rate of "sudden unexpected infant deaths"—the broad category that includes suffocation and strangulation victims, infants whose deaths seem due to SIDS, and baby deaths whose cause is unknown—has been dropping in the United States since about 1991, roughly when warnings began to appear about the dangers in face-down sleeping. The rate fell for six or seven years and then reached a plateau at about 3,800 such deaths annually, roughly what it was by 2004, when data collection for the study ended. During the last eight years of the study period, the percentage of deaths attributed to SIDS dropped to about 60 percent, and the deaths attributed to suffocation or strangulation rose to about 13 percent.
For the study, the cause of death was taken from the death certificate and was based on autopsy results or findings of the death-scene investigation. We don't know (but can guess) the details. A dead baby found under an adult in the morning or with mouth and nose blocked by a lot of soft bedding was probably thought to be a suffocation victim. A baby found with no apparent cause of death and nothing nearby to block breathing was likely thought to be a victim of SIDS. A baby found on a soft bed with nearby bedding but no evidence for suffocation was probably assigned to the "unknown" category. There could well have been some misassigned deaths, but between autopsy results and the standardized death-scene investigation methods provided by CDC, it's not likely that many of the cases thought to be due to suffocation or strangulation were really caused by SIDS.