Unexpected reboot OS6360-P24

[mod]

hello,

I'm running into issues with 2 OS6360-P24. One is slave of virtual-chassis (2 members) and second is master (1 member).

These switches are connected to UPS but they rebooted without any reason.

From master, I see in "show log events" :CMM : ChassisSupervisor : The switch was restarted by a power cycle or due to some type of failure

From slave, "show log events" does not works on slave but I see "CMM : vcmCmm : Virtual Chassis: Chassis 2 status changed to Runtime-Left" I suppose slave rebooted

Both switches are using 8.10.105.R02 GA firmware

How to diagnose unexpected reboot ?

Thanks in adavance,
Regards