I can't say exactly what happened as I was not there, but looking at the photos I have found, I have my suspicions. The position of the base section is the biggest clue. The angle of elevation of the base section is much lower than it would have been to get the bucket to the rooftop. There are only two possible ways for that section's elevation to lower, either hydraulic failure or operator error.
With the aerial elevated to where the bucket was higher than the roof, the weight of each aerial section above the base section keeps them in alignment. If the bucket is lowered to just rest on the roof, the sections should maintain their alignment. The manufacturer does not recommend this practice, instead, one should maintain a small gap between the bucket and roof and let personnel embark/disembark without the aerial making contact. If downward force was applied to the aerial and the bucket made contact with the roof with enough force, the pictured results could happen. A hydraulic failure with contact from a short distance away should not cause this to happen and the story reports that the bucket was only a short distance above the roof when the failure occurred.
The investigation will be enlightening!