How on earth did this happen? Never seen this before?

Yes but if you drive accordingly to road conditions, this wouldn't happen?

I may be wrong, but this is quite a common failure.
The tube brakes due to torsion (fatique). Reason is the open side at the front making the tube an open section which is by far not a strong for torsion forces as a closed section.
For this reason Citroen advices to weld a U section on the back of the steering rack to create a closed section to take the load of the open part.

Clear?
Cheers, Bart

Bart,
this looks to me more like a failure of the cross beam in bending.
Because the arm is mounted to the cross beam using a (relatively) friction free bearing, the load on the beam will be the resultant of the vertical reaction at the pivot and the horizontal pull in the tie rod.
Since both of those loads are being fed onto the cross beam through the bearing, there's no way that they can exert any torque on the beam.

My guess is that the resultant load would be acting as though it were trying to pull the outer end of the beam backwards and upwards, towards the passenger seat.

A possible trigger for the failure would be if the axle bolts had worked loose.
The bolted joint between the beam and the chassis would then behave as a pinned connection ( rather than a restrained one), so the bending load would be transferred onto the weaker 'open' section of the beam instead of being concentrated on the short cantileverered outer section where the bearings are mounted.

Comments on a plain beermat to the usual destination...