The watchdog timer on RB products are all hardware watchdog timers, the CPU can be hard locked and the watchdog timer will still reboot the board. The RB software has to keep poking at the watchdog timer located in a PLD before expiry or it will reboot the CPU or the PLD is doing the querying, not really sure.
So, what could be the cause?
Hard to diagnose. Check the settings like SNMP on routerboard OS seems quite flakey and can drive the CPU to 100% for no good reason. off site logging can help. Are you using the "UP" part of the board to power the ubnt access points or using the included injector?
Turning it off will likely stop your problem, but one of the things I love about routerboards is the hardware watchdog and has saved me a few late night power cycles.
You can use any public NTP server to set the time on an RB.--
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