Whay is CCD
Explanation is simple. I have had those cases enough. And in that natural hive envitonmrnt CCD moves from hive to hive.
When varroa is far enough, all brood will be violated. When brood emerges, new bees live perhaps a week and then they die. After that older summer bees will die for their age, and the hive will be empty
Another hive may notice that varroa hive can not resist any more robbers, and they start robbing. Robber hive may get dedly dosage of mites in couple of weeks even if they were normal before robbing.
Many may think, that what a wonderfull world such natural fairytale forest, but it is far from that.
Robbers can fly long distances and carry mites to their home.