It's always interesting to read contemporary accounts. Lt Col G E Badcock (A history of the transport services of the Egyptian Expeditionary Force 1916-1918) simply says (p.249): Anti-Aircraft Section used Thornycroft lorries to a small extent, but so few were employed that it is impossible to criticise this vehicle.
However, Lt Col F W Leland (With the MT in Mesopotamia) says (p.113): There were several Anti-Aircraft sections and 'Caterpillar' companies which also took part in this advance to Baghdad. The 'Caterpillars', however, were unable to move owing to the fact that no bridges were able to carry their weight at the time, and they were eventually brought up by river. The Anti-Aircraft sections, notwithstanding the extraordinary heavy going over deep sand, managed to get through to the Dialah by dint of pushing and towing, etc. Most of them suffered a fair amount of damage, the dumb-iron and Carden shafts being a source of trouble. With regard to the frame, it was considered that the position of the towing hook was too low down, and these were afterwards placed at the bend of the dumb-iron, in a straight line with the rest of the frame.
And then later (p.183): THORNYCROFT ANTI-AIRAFT GUN LORRIES: The front towing hooks of these were altered and placed higher up just on the bend of the dumb-iron, so that a straight pull could be obtained instead of the original position, which was inclined to have a lifting action. This was found necessary in view of the number of dumb-irons which were smashed in towing these heavy vehicles over almost impassable stretches of desert.
So perhaps the fracture is a result of towing?