Why did Trisquel installation fail when not connected to the Internet?

I tried to install Trisquel via the installer and had no form of Internet connection enabled. While grub2 was installing, I got the following error:

"GRUB Installation Failed. The 'grub-efi-amd64-signed' package failed to install into /target/. Without the GRUB boot loader, the installed system will not boot."

However, when I tried to install it again, this time with Internet enabled and the "Download updates while installing Trisquel" option enabled, I did not encounter the problem again and the installation was successful.

Anyone know why this could be?