The video appears to show the aircraft being towed in a series of more ‘jerky’ movements until the nose undercarriage retracts and causes the tow bar shear pin to do its job and break at a pre-determined load. The resultant investigation will determine If the cause was a tug problem or aircraft brake problem (possibly intermittent anti-skid) If serviceable aircraft brakes were ‘on’ I doubt the tug would have been able to move the aircraft.
I understand that a geometric mechanical over-centre mechanism positively locks most undercarriages in the down position and most usually on the ground undercarriage ground lock pins are fitted to prevent the mechanism from having any possibility of unlocking.
If the incident required the aircraft to vacate the runway soonest perhaps the haste omitted safety pin(s) being fitted and for anyone riding the brakes that nasty ‘thud’ would have focused the mind they were sitting on a proficient ejection system.