The operator is at fault, but so is the business. Obviously I don't know what is really happening at this business, but in my experience these accidents occur because of unsafe activities that have historically been allowed to occur (by management). I think you'll find this is the actual root cause of most industrial accidents.
The accident occurred due to an operator driving with an unstable load. My guess is that this is not the first time an operator has driven with an unstable load, but operators probably aren't being disciplined for driving with unstable loads unless the load falls and hurts someone or causes damage.
So yes, the operator is at fault because he is operating a lift truck with and unstable load and his training should have clearly shown this to be unacceptable. However, if the company doesn't enforce safe operating procedures on a day-to-day basis (as most don't), they are responsible (I would actually say they are more responsible) for basically making unsafe activities an acceptable part of day-to-day operations. You need to deal with unsafe activities immediately as they are observed, not wait until someone gets hurt, then start pointing fingers.