I’m not sure if you are stating this to indicate that it recovered from any errors on its own, or if the startup on its own was unexpected.
You can set the behaviour, if you are asking that, and are not aware of the options.
I do have to post a warning though, that even if rare, power outages are not good for electronics. This could easily cause disk corruption, disk failure, or complete NAS failure. If your data is of value to you, and you don’t want a bad surprise one day, I hope that you have an appropriate backup strategy in place. Not just for power outages, but for accidental deletion, malware, etc as well.
You should also check the integrity of the data after an outage. The admin UI should be prompting you to perform system a file system check after these type of events. While this is guaranteed to recover any lost data or find all the potential issues, it would verify some integrity of the volume and would be a really good idea to run. You may also want to closely monitor the drives for SMART errors and the like to see if they show any negative trends.
Ideally of course, a UPS would allow for an orderly shutdown on these rare events.
Your data, your money, your choice.