weird df -k output

[Bashar]

FreeBSDers,

got this weird df -k output on one of my 5.3 boxes which show either 
after long uptime or high load:
# df -k
Filesystem     1K-blocks     Used    Avail Capacity  Mounted on
/dev/ad0s1a       507630   215166   251854    46%    /
devfs                  1        1        0   100%    /dev
/dev/ad0s1g    102405926 49029352 45184100    52%    /home
/dev/ad0s1d       507630     9832   457188     2%    /tmp
/dev/ad0s1e      5052462  3957240   691026    85%    /usr
/dev/ad0s1f      5052462  1796916  2851350    39%    /var
procfs                 4        4        0   100%    /proc
/libexec          507630   215166   251854    46%    libexec
/lib              507630   215166   251854    46%    lib
/usr/lib         5052462  3957240   691026    85%    usr/lib
/usr/sbin        5052462  3957240   691026    85%    usr/sbin
/usr/share       5052462  3957240   691026    85%    usr/share
/usr/bin         5052462  3957240   691026    85%    usr/bin
/usr/man         5052462  3957240   691026    85%    usr/man
/usr/X11R6       5052462  3957240   691026    85%    usr/X11R6
/usr/libexec     5052462  3957240   691026    85%    usr/libexec
/usr/local/bin   5052462  3957240   691026    85%    usr/local/bin
/usr/local/lib   5052462  3957240   691026    85%    usr/local/lib
/var/spool       5052462  1796916  2851350    39%    var/spool
/var/lib         5052462  1796916  2851350    39%    var/lib
/var/run         5052462  1796916  2851350    39%    var/run
/var/log         5052462  1796916  2851350    39%    var/log
/home/b12 102405926 49029352 45184100    52%    b12
/tmp              507630     9832   457188     2%    tmp
/dev                   1        1        0   100%    dev
/bin              507630   215166   251854    46%    bin
/proc                  4        4        0   100%    proc

and it disappear right after the reboot, dunno why its mounting several 
dirs as partitions?
also mount output as follows:
/dev/ad0s1a on / (ufs, local)
devfs on /dev (devfs, local)
/dev/ad0s1g on /home (ufs, local, soft-updates)
/dev/ad0s1d on /tmp (ufs, local, soft-updates)
/dev/ad0s1e on /usr (ufs, local, soft-updates)
/dev/ad0s1f on /var (ufs, local, soft-updates)
procfs on /proc (procfs, local)
/libexec on libexec (nullfs, local)
/lib on lib (nullfs, local)
/usr/lib on usr/lib (nullfs, local)
/usr/sbin on usr/sbin (nullfs, local)
/usr/share on usr/share (nullfs, local)
/usr/bin on usr/bin (nullfs, local)
/usr/man on usr/man (nullfs, local)
/usr/X11R6 on usr/X11R6 (nullfs, local)
/usr/libexec on usr/libexec (nullfs, local)
/usr/local/bin on usr/local/bin (nullfs, local)
/usr/local/lib on usr/local/lib (nullfs, local)
/var/spool on var/spool (nullfs, local)
/var/lib on var/lib (nullfs, local)
/var/run on var/run (nullfs, local)
/var/log on var/log (nullfs, local)
/home/b12 on b12 (nullfs, local)
/tmp on tmp (nullfs, local)
/dev on dev (nullfs, local)
/bin on bin (nullfs, local)
/proc on proc (nullfs, local)


any idea what is this and what could be causing it?