Just decided to share this hoping it might help others experiencing the same.
I left my room while ESO was installing only to return later and find out that it has rebooted.
After searching the web for a bit and going through the event logs, I found out the restart was caused by a Windows Update (probably a more critical one since it couldn't wait for my input).
Initially was a bit more concerned it might've been some sort of crash or something but was just the Windows Update.
So if you want to avoid having a reboot during ESO installation you should stick with your PC so you can act on a possible Windows Update request (postpone, for example).
All the best and have fun!