su rails session mysteriously closes by itself

I need help troubleshooting a strange problem. When I ssh into a server I have and do an su rails from root user the session closes by itself:

root@ubuntu-server:/4# su rails
rails@ubuntu-server:/4$ exit

I'm not typing in exit...it just appears by itself ...it may last more than one minute but it always closes within the first 5 seconds of a new minute, as seen below...how can I troubleshoot this?

from /var/log/auth.log:

Mar 24 19:03:13 ip-10-149-16-81 su[11304]: Successful su for rails by root
Mar 24 19:03:13 ip-10-149-16-81 su[11304]: + /dev/pts/2 root:rails
Mar 24 19:03:13 ip-10-149-16-81 su[11304]: pam_unix(su:session): session opened for user rails by root(uid=0)
Mar 24 19:04:01 ip-10-149-16-81 CRON[11317]: pam_unix(cron:session): session opened for user root by (uid=0)
Mar 24 19:04:01 ip-10-149-16-81 CRON[11318]: pam_unix(cron:session): session opened for user root by (uid=0)
Mar 24 19:04:01 ip-10-149-16-81 CRON[11319]: pam_unix(cron:session): session opened for user rails by (uid=0)
Mar 24 19:04:01 ip-10-149-16-81 CRON[11320]: pam_unix(cron:session): session opened for user rails by (uid=0)
Mar 24 19:04:01 ip-10-149-16-81 CRON[11318]: pam_unix(cron:session): session closed for user root
Mar 24 19:04:02 ip-10-149-16-81 CRON[11320]: pam_unix(cron:session): session closed for user rails
Mar 24 19:04:03 ip-10-149-16-81 CRON[11319]: pam_unix(cron:session): session closed for user rails
Mar 24 19:04:03 ip-10-149-16-81 su[11304]: pam_unix(su:session): session closed for user rails
Mar 24 19:04:04 ip-10-149-16-81 CRON[11317]: pam_unix(cron:session): session closed for user root