LiveUSB booting fails strangely on Dell XPS 13

My machine is a Dell XPS 9343 with ubuntu 16.04 installed (64 bit), memory 7.7 GB. It's a machine with loads of proprietary drivers which shipped with ubuntu installed by the manufacturer.

I created a liveUSB of Trisquel 11 using different software: with Startup Disk Creator, with mkusb and with Etcher. I tried with different USB pens.

In GRUB, I choose to boot from the USB pen and I get to a Trisquel homescreen (mountains and the trisquellogo superimposed at the centre of the screen), then a black screen and finally I enter the OS.... except that it's the wrong OS! It is ubuntu 14.04. If I choose to log off, I am greeted with "Goodbye, trisquel".

This is really odd.

Can anyone help?
I tried creating a liveUSB of another OS, SparkyLinux, and that works!

Thank you