There could be a few things going on to cause this. This could be a drone eviction. If you notice bees being actively removed check to see if the bees on the ground are drones. Drone evictions happen in the late summer and fall to get the hive ready to overwinter (i.e. reduce the number of mouths to feed).
Pesticide spraying can sometime cause events like this as well. If any of your neighbors, or the city, have recently sprayed pesticides or insecticides wind drift can carry the chemicals to your hive. Foraging bees are the most effected in this case. Often spraying results in hundreds if not thousands of dead bees. If you suspect spraying than you need to make sure to cover your hive when the spraying is taking place. Unfortunately there is not much you can do but wait and see after the spraying event. Depending on the chemicals used some colonies will pull through and some will not.
Tracheal mites can cause bees to crawl around the hive because it weakens their tracheal system (breathing system) to the point that they are unable to fly. The only way to diagnose tracheal mites is with a dissected bee under a microscope, these mites are too tiny to see with the naked eye. There are several treatments available, one being menthol crystals, and the use of shortening and sugar patties to mask the smell of young, newly emerge adults bees that female tracheal mites seek out. Most beekeepers today try and breed bees with tracheal mite resistance. Raising tracheal mite resistant colonies seems to be the best option in the fight against tracheal mites. Splitting healthy, treatment-free, robust colonies may be one of the best ways to defend against this mite.