UDM-Pro: Mystery auto-shutdown

My UDM-Pro tried to reboot itself while I was away for a few days and failed. Instead I had the ‘It is safe to poweroff’ display indication greeting me when I returned. This was in the log:

Oct 24 00:15:18 UDM-Pro user.emerg kernel: [60605.093953] reboot:   called by: [31271]/bin/busybox poweroff
Oct 24 00:15:18 UDM-Pro user.emerg kernel: [60605.099820] reboot:   called by: [31269]/bin/sh /sbin/ubnt-systool poweroff lcm
Oct 24 00:15:18 UDM-Pro user.emerg kernel: [60605.107213] reboot:   called by: [ 1876]/usr/bin/ulcmd
Oct 24 00:15:18 UDM-Pro user.emerg kernel: [60605.112666] reboot:   called by: [    1]init
Oct 24 00:15:18 UDM-Pro user.emerg kernel: [60605.116977] reboot:   called by: [    0]

Clearly invoked by /usr/bin/ulcmd but I don’t know what that does and how I should try to prevent a recurrence. Anyone seen this kind of behaviour?