Servers can reboot for any reason. There are tons of kernel issues, especially since Meltdown & Spectre, that cause machine reboots in Linux especially on high traffic machines.

I've worked in production environments with thousands of machines and random reboots are a completely normal event for some workloads. Combination of hardware issues & kernel issues with hundreds of thousands of lines of code makes it inevitable. I would be surprised if NordVPN even noticed and their architecture wasn't designed to automatically start everything at boot.

You can't be perfect at scale - you just need to design your work loads to be redundant and fault tolerant.