This seems to be due to the MCU watchdog getting deactivated while the mv88f6281 watchdog gets initialized. Along with all the other hardware support, they patched this into the 2.6.31.8 kernel:
$this->bbcode_second_pass_code('', '
#if defined(CONFIG_ZYXEL_NSA325)
/* MCU tries to reset CPU every 1 minute for avoiding system hang before watchdog timer is enabled.
* If watchdog timer is enabled, then set GPIO 14 to tell MCU that do not reset CPU.
*/
regValue = MV_REG_READ(0x1010);
regValue |= BIT14;
MV_REG_WRITE(0x10100, regValue);
#endif
')
So the system is forced to reset if the mv88f6281 watchdog is initialized too late