Many theories are floating around. The obvious one was cellphones, because cellphones are the hobgoblin of modern society, much as irradiated food was decades ago. Another theory is that some sort of fungus from overseas is attacking them. But whatever the cause, they simply aren't finding corpses of the bees. So unless the bees have suddenly gone cannibalistic, there is only one rational explanation:
Terrorism.
There are hundreds, perhaps thousands, of sleeper cells spread around the country. Each one has purchased an old flower shop or gardening store, and has closed it down. By using pheromones or cellphones or what not, they are attracting the bees, and trapping them in the stores, where there is ample short-term protofood for them. Meanwhile, they have dug a hole from the store down to some subway, shopping mall, government building, or whatever target is most convenient -- stopping just shy of breaking through into the target structure.
When the time is right, two suicide bee-ers will engage in their plan. One will go to the target with a pickaxe (or perhaps just cleats), open the passageway, and start spraying a light sugar/pollen solution over everyone nearby. The other will aggravate the bees and use smoke or some other mechanism to force them down the hole.
So remember: the next time you walk by a flower shop and they are closed, call Homeland Security. Better safe than sorry. Eternal vigilance and all that. The life you save might bee your own!