Purely a hypothesis ...
During pushback/tow, the tug lost traction and with wheels spinning the front of the cab hit the towbar (pivot on the pin connecting tug to towbar) and caused the bar to bend as under tension during operations. Bar bent in a similar way to standing on an empty drink can and touching the side of the can, lateral push force on its own probably wouldn't cause bar to bend.
Guessing the aircraft moving forward and the bent bar didn't create enough force for shear pins to break (still connected to aircraft) and as the bent bar allowed the aircraft to move forward over the tug, friction from tug tyres sideways meant the tug was trying to 'roll' so the suspension compressed and the opposite side of cab moves up (possibly in conjunction with the aircraft pushing against cab).
Others have said tug (possible max weight 54T from earlier link?) was not up to the job, could this be the route cause and knock on effects? Loss of traction due underweight tug, wheels spin and tug moves sideways, tug hits bar and bar bends, and aircraft forward momentum continues to bend bar and aircraft hits tug.
Interesting that Menzies bar used and Swissport tug. Who handles China Southern in LAX? Was the equipment borrowed following break down and not really fit for purpose?