Four machines have rebooted the same time. Why?

Jan  9 11:30:23 atlas076 ntpd[7082]: synchronized to 192.33.218.1, stratum 2
Jan  9 18:47:30 atlas076 kernel: Uhhuh. NMI received for unknown reason 0c.
Jan  9 18:47:30 atlas076 kernel: Do you have a strange power saving mode enabled?
Jan  9 18:47:30 atlas076 kernel: Dazed and confused, but trying to continue
Jan  9 18:52:34 atlas076 syslogd 1.4.1: restart.
Jan  9 18:52:34 atlas076 kernel: klogd 1.4.1, log source = /proc/kmsg started.
Jan  9 18:52:34 atlas076 kernel: Linux version 2.6.18-308.24.1.el5 (mockbuild@lxdist04) (gcc version 4.1.2 20080704 (Red Hat 4.1.2-52)) #1 SMP Wed Dec 5 09:45:59 CET 2012

run command: ssh -l root atlas077 grep -3 Dazed /var/log/messages
Jan  9 14:24:25 atlas077 pbs_mom: LOG_ERROR::No such process (3) in mom_get_sample, 18799: get_proc_stat
Jan  9 18:43:33 atlas077 kernel: Uhhuh. NMI received for unknown reason 1c.
Jan  9 18:43:33 atlas077 kernel: Do you have a strange power saving mode enabled?
Jan  9 18:43:33 atlas077 kernel: Dazed and confused, but trying to continue
Jan  9 18:48:35 atlas077 syslogd 1.4.1: restart.
Jan  9 18:48:35 atlas077 kernel: klogd 1.4.1, log source = /proc/kmsg started.
Jan  9 18:48:35 atlas077 kernel: Linux version 2.6.18-308.24.1.el5 (mockbuild@lxdist04) (gcc version 4.1.2 20080704 (Red Hat 4.1.2-52)) #1 SMP Wed Dec 5 09:45:59 CET 2012

run command: ssh -l root atlas078 grep -3 Dazed /var/log/messages
Jan  9 18:31:59 atlas078 pbs_mom: LOG_ERROR::No such process (3) in mom_get_sample, 1340: get_proc_stat
Jan  9 18:42:06 atlas078 kernel: Uhhuh. NMI received for unknown reason 1c.
Jan  9 18:42:06 atlas078 kernel: Do you have a strange power saving mode enabled?
Jan  9 18:42:06 atlas078 kernel: Dazed and confused, but trying to continue
Jan  9 18:47:09 atlas078 syslogd 1.4.1: restart.
Jan  9 18:47:09 atlas078 kernel: klogd 1.4.1, log source = /proc/kmsg started.
Jan  9 18:47:09 atlas078 kernel: Linux version 2.6.18-308.24.1.el5 (mockbuild@lxdist04) (gcc version 4.1.2 20080704 (Red Hat 4.1.2-52)) #1 SMP Wed Dec 5 09:45:59 CET 2012

run command: ssh -l root atlas079 grep -3 Dazed /var/log/messages

run command: ssh -l root atlas080 grep -3 Dazed /var/log/messages
Jan  9 17:42:26 atlas080 pbs_mom: LOG_ERROR::No such process (3) in kill_task, 24788: get_proc_stat
Jan  9 18:41:21 atlas080 kernel: Uhhuh. NMI received for unknown reason 1c.
Jan  9 18:41:21 atlas080 kernel: Do you have a strange power saving mode enabled?
Jan  9 18:41:21 atlas080 kernel: Dazed and confused, but trying to continue
Jan  9 18:46:24 atlas080 syslogd 1.4.1: restart.
Jan  9 18:46:24 atlas080 kernel: klogd 1.4.1, log source = /proc/kmsg started.
Jan  9 18:46:24 atlas080 kernel: Linux version 2.6.18-308.24.1.el5 (mockbuild@lxdist04) (gcc version 4.1.2 20080704 (Red Hat 4.1.2-52)) #1 SMP Wed Dec 5 09:45:59 CET 2012