According to the man page for usermod, this is what the -G option does.
If the user is currently a member of a group which is not listed, the user will be removed from the group. This behaviour can be changed via the -a option, which appends the
user to the current supplementary group list.If you had used the -a option, this wouldn't have happened. One way to fix it would be to boot from a live CD and chroot into your Trisquel partition.
