He struck the top beam of the second layer stack, and that layer caved in.
Indeed.
Strength of many structures relies on the geometry distributing the load. Mess up the geometry and all of a sudden the load is focussed at a point. Just brushing that front beam would be enough. A few tonne of forklift on one side and a few tonne of goods on the other - and that beam had no chance.
Once buckled, it had no ability to support the load, so it was pushed down at that point and since the ends are tied into the uprights - the uprights were pulled inwards, destroying their ability to support the goods.
If the racking was just a series of independent sets running down the aisle, then one could expect only one set of racking would collapse - but it would seem these were all linked together, so the whole aisle would be affected.
This, in itself, would indicate the racking was overloaded - but (to me) the real indicator of this is that other aisles collapsed as the result of things tumbling into them.
So, while the forklift operator did make a mistake, the whole place was a house of cards - just waiting ......