Do not direcly point to the manufactorer. What is the history of the pump. Maybe the back rigger was not completley out you never know whit some people.

http://concretepumping.com/index.php?mode=newboard&act=topic&tid=10513

In most cases a catastrophic failure of any kind of structure normally exhibits early signs that something is not quite right in regards to its designed purpose. Defects in a structure can be caused by mostly missues by the end user, fatigue, or even forces that acting on a structure that were failed to be considered or underestimated in the design phase.

In this situation, maybee the operator short rigged that particular outrigger in the past and overloaded it in a non reinforced section of the outriger housing. Off couse this may have been many months or years ago. 

Good practice is to keep your machinery clean , train your operators while they wait for mud just to have a look around now, report defects and very importantly get your machinery inspected annually at the very least by a competent person who has familiarity in concrete pump designs.

Even then there is still a very minute chance things can go pear shaped. But if you can take precaustions your machine stands a better chance of maximising and sometimes exceeding its design life.

don't know if it's the same unit