Your left rear motor failed to provide thrust.
You were flying Solo Firmware 1.3.1 - very old, dangerous ESC signal voltage level bug bit you.
Make sure to update to 1.5.4 at least. https://github.com/OpenSolo/ardupilot-solo/releases
( or current ArduCopter build - if you replace the cube, or modify your own with madhacker.org 5v mod)
The easiest way to upgrade to proper version is to run Solex (Android app) , and ask it to install OpenSolo (which fixes everything for you.)
