There can be many bees from many hives at a food source, so the behaviour is not defending the other bees but chasing away intruders. Why just this one bee? Bees from different hives have very different levels of aggression, this being determined by the genes of the queen (queens are deliberately bred for docility). One of the closer and more aggressive bees identifies you as a possible ‘intruder and nectar thief’, and buzzes you accordingly. The bee is only curious and giving a warning: if it really thought there was a threat it would immediately sting. Does this reduce the behaviour of the other bees? No, quite the contrary. Once one bee starts being aggressive, especially once stinging starts, the smell excites many others to join the battle. However, bees are usually not aggressive at foraging: they reserve that behaviour for guarding the hive and in that location there is indeed selection of duties, in this case by age. Younger bees are ‘house bees’.   

How do bees know where to attack an enemy? They have good eyesight, including at low light. They can probably detect eyes. They also have good smell, and will go for skin. They also smell fear.  Fortunately, for most bee strains (other than the African) you are unlikely to get many foraging bees in this frame of mind.

At a mixed feeding party there will often be some squabbling between the bees. Also, they have to fight off wasps and other invaders. African bees are a type that is particularly aggressive and will attack people even while foraging, whereas Italian bees for example will seldom attack in the same situation.