Only the females can sting. The males do not have a stinger.
Carpenter bees, which look a lot like bumblebees, are reported to actually charge and chase people and other animals, bumblebees not so much.
In the fall, with the cooler weather, at least in places with 'winter', the thing that is going on is survival of the species--the old queen, all the males and workers will die, only the new queens survive to start over the next spring.
Depending on where you are at in *their* life cycle...
...when the new queens are still in the nest, the males and the workers are very protective of the nest, and become much more aggressive about it to ensure the survival of the new queens.
...once the new queens have hatched and ventured off to find a new winter hibernaculum, they are not so obsessed with protecting the now old nest, but the new queens are themselves, rather aggressive about protecting themselves.
It is possible that you have a nest somewhere near that part of the garden that you are unaware of, or you may have encountered a new queen.
For the most part, they will fly at you and generally skim your head until you get the message and back off; they don't sting unless you initiate contact with them by swatting or grabbing them or in the case of the dogs, snapping at them (unlike the social wasps, which will swarm you, land on you, and sting you repeatedly as they crawl under your clothes to sting you better!).
