Hello Jake and welcome to the forum.
Reserving your copy of Windows 10 from a Windows 8.1 machine should not have resulted in any download, installation and or reboot.
Perhaps there were some other downloads, software installations, drivers, or configuration changes that were performed at or near the same time??
You should be able to, at minimum, boot from your installation media and select the "repair" option and perform a system restore back to a point in time preceding the problem's first occurrence