Aggression is mislabelled as unpredictable when it is not. A dog always shows signs of its discomfort with a situation before it resorts to displaying an aggressive behaviour. Unknowledgeable humans either don't see the signs or misinterpret them and assume the dog is acting unpredictably. And there are dogs who have a great prey drive.
This behaviour is called predatory drift. It is not aggression, but it looks like it because it is often preceded by a prey drive reaction. To understand predatory drift, you first have to understand aggression and prey drive and the difference between the two.
Aggression, simply explained, is the expression of a behaviour with the intention of self-preservation. This means that a dog somehow feels threatened by an external stimulus and has decided its other two options - flee or freeze - will not achieve its goal of self-preservation.
Dogs prefer to first use defensive options, such as retreating from a threat. When that does not work or is not an option because the dog is confined, it will freeze in hopes that the threat will leave. If those reactions don't work, it will choose the final option, which is no longer defensive but offensive, and assault the threat to make it leave.
Prey drive is a dog's instinctual need to chase and catch things. This is a fairly common behaviour to witness. A dog that loves to play fetch, chase squirrels or cats has a strong prey drive. A dog that lifts its head to watch a ball roll by does not have a big prey drive.
When wolves passed their DNA to dogs, prey drive was part of that DNA. Prey drive is what motivated wolves to chase and catch their meals. Since it is a survival trait, it is a huge part of their genetic makeup and impossible not to have been passed down to dogs. Prey dive is not specific to a breed. Prey drive relates to personality and genetics rather than breed.
The majority of dogs with a high prey drive will enjoy chasing a squirrel, but are not likely to kill or even injure the squirrel if the opportunity presents itself. This is due to domestication and breeding practices that have softened the predatory aspect of the prey drive.
Predatory drift is a glitch in the system, so to speak, and I want to stress that it is not aggression, although it often gets labelled as such by those unfamiliar with it. Predatory drift is when a dog gets aroused by the high-pitched sounds, the energetic struggles or the frenetic behaviour of an animal or even a person in distress and becomes predatory towards it. The dog focuses with great intent on the object of its arousal.
Predation sounds like a pretty scary word and can create fears about aggression, but it is not the same. Predatory drift is just that - a drift from prey drive to predatory. It becomes dangerous when the object of predation is small. Dogs may have lost the kill sequence of the predatory act, but a great size and strength difference can result in death very easily. This is not an act of aggression, but the expression of a latent aspect of the prey drive.
The limited research on the subject suggests that any dog has the potential to drift from prey drive to predation. Dogs that have never shown any predatory proclivities, past aggression to other dogs and even dogs that are considered well-socialized can display it. It happens in an instant and is generally not preventable if it has not been witnessed in the past. This is where it gets its "unpredictable aggression" label from.