Q1230010: Why are remote sessions not closed ?
This can happen when the command started is a background process which keeps current stdout or stderr open. The solution is to redirect stdout and stderr. Click here for more information.
The other possibility is that you are running a version of OpenSSH which is older than 4.9. These versions contain a bug which causes it to never close sessions for root users. More information in the release notes for version 4.9 under (bz#926).