Aberrant Behavior Syndrome describes aggressive behavior towards humans, which can result in serious injury and possibly death. It is not common, but can affect both male and female alpacas that have been hand reared. It appears to be more prevalent when alpacas are hand reared in isolation from other alpacas.
It can best be explained by understanding that alpacas and llamas are herd animals. They have an established hierarchy within herd, and this is often determined by fighting for supremacy in the herd. Young alpacas are generally put in their place by the older animals in the herd. As males reach maturity they fight to assert their own dominance. This fighting includes charging, biting, stomping, spitting and screaming at males as they see as a challengers in the herd hierarchy.
In situations where animals are raised in isolation from the herd, they still have the instincts to assert domination, the herd becomes the humans that have raised them, so therefore these animals can attempt to assert dominance over the humans in the herd, using the same methods of fighting they would employ with other alpacas. In rare cases females can also display this type of behavior.
In a herd situation alpacas can inflict serious injuries on each other, however they are rarely fatal. However if alpacas attack humans in a similar manner injuries could be very serious, and children are at particular risk. Responsible alpaca and llama owners will not sell baby alpacas to be hand reared because of the potential risk to the safety.
Many herd animals that are hand reared can display similar types of behavior towards humans, llamas and alpacas can be particularly dangerous because they have the potential to inflict serious injury.
I have not personally seen an actual instance of ABS because most alpaca and llama breeders take this risk seriously, but there are people that will ignore all of the advice they are given.
There are a number of Youtube videos showing just how dangerous this behavior can become.
It is more common for hand reared alpacas to be spoiled by owners, which often results in the animal not having respect for the humans that own them. This behavior is annoying, but it is possible to retrain these animals, however in the case of ABS, it is unlikely that behavior can be modified.