Feral bees are subject to nature's ailments: varroa, nosema, acarine, viruses and more. The selective pressure on wild bees is for reproductive success and survival, a heavy element of which is dictated by the ability to resist disease. Hardy bees will tend to be selected.
Commercially kept bees are to some degree insulated from disease. The selective pressure imposed by beekeepers is directed towards honey-making. Productive, unstingy and unswarmy bees will tend to be selected.
Diseases borne by feral bees will, from time to time, transfer to the commercial population. The disease variants will tend to be vigorous (they have contended in an arms race with the much hardier feral bees), and, on some occasions, entirely new to the commercial population.
The result is CCD.
We are not brought down except by ourselves.