Reason could be many Eddie, pin pointing the reason in a feral colony isn't possible unlike a managed colony in a hive.
Most commonly is it down to poor genes/cross breeding in the queens. When they re-queen , they will either swarm with the old queen and leave a new queen to emerge or will superscede the old queen and eventually bump the old one off. The new daughter queen will have 25 - 50% of the mothers genes the rest comes from the drones she mates with when on the wing , she will leave the nest for about an hour. Sometimes cross breeding with other sub species of Apis Mellifera, the hybrid queen then is no longer an F1 pure bred and is likely to be an F2 or F3 hybrid with sometimes bad traits arising from the bad genes she inherits via mating. These can be aggression/defensive behaviour, following and possibly stinging. Back in the colony her pheromones has a part to play in the colony temperament, any bad genes shows up when she produces the future brood she lays.
A colony that becomes queenless for what ever reason will become tecthy/agitated. If a new queen is taken by a bird during the risky time she is away from the colony whilst mating, this can leave the colony in serious trouble esp if there isn't a backup plan within the colony.
They may just feel threatened and are defending the colony/territory as to speak, wasps are about now and will try and plunder them and the larvae for the meat content as they will be raising.
They can be protective of collected stores or tecthy if nectar flow has dried up.
Perfumes, after shave ,deodorant's and hair products can also affect them and cause alarm to them as the chemicals used in these products mimic their own alarm pheromones.