Shenyang, China. It's not often that we get full reports on why accidents happen from anyone that investigated an accident. I ran across one that even though it's a translated article, it does a decent job of telling the story. It comes from the Safety Research Institute of Liaoning Province, Shenyang with the full report being found here.
A QTZ315 Chinese made tower crane was being climbed down in good weather. It collapsed with 5 workers on the crane. The report doesn't indicate the fate of them all, but given a nearly 100 foot fall and the volume of blood on the site, the prognosis isn't good. The crane was found to have what's listed as an "ash bucket" (concrete?) on the hook and a tower suspended on the armature. The accident investigators found that the "claws" (either resting dogs or climbing dogs) failed structurally due to what is called a scratch and uneven loading. The failure caused the superstructure fall and crash into the tower. At this point the lowest mast section just above the knee brace section collapsed due to the vertical overload. The crossbase stayed in place and the rest of the crane fell over.
Looking at the pictures of the "dogs" it would appear that it simply wasn't in far enough. I think that the report is trying to indicate the same, but it's lost in the translation. There is an armature that is also fractured and that's likely due to the failure and subsequent slamming into towers and lacings as the superstructure came down. In my experience in climbing, it's 3 guys trying not to sleep for 25 minutes between sections while one guy makes sure that the lower dogs are in tight with each stroke and a technician runs the pump. Buy always, you have to make sure that the dogs are in tight. They are only designed to work if they are all of the way in and even if they aren't all of the way in and can take the load, they can slip in with 300,000 lbs of load on them, and that isn't good for any structure.