When you say no obvious trigger, dies the dog just turn on people out of nowhere? Or do you mean the resource that the dog is guarding changes constantly?
Can you snap the dog put of it, does it go on for more than just one snap?
Does the dog seem disorientated after?
If your dog attacks out of nowhere, with no trigger, it's a lengthy attack (more than a few seconds) and your dog seems disorientated after, there's a chances it's rage syndrome. It uses to be called cocker rage, as it was thought to impact cockers and not other dogs.
Rage syndrome is a neurological disorder. Similar to epilepsy. Except the fit stage is where the dog glazes over and becomes extremely vicious. You can't stop them until it's over.
Good cocker breeders spent alot of time breeding it out of cockers and its known to impact more breeds now. A friend of mine had an English bull terrier with it. There's been some attempts to treat it with epilepsy medication but in most cases the dog needs to be put to sleep. It can't be trained out of a dog. If its trained out, it wasn't RS.
If there's a pattern to it, it's unlikely to be rage syndrome. I am not posting this to upset you. Just to make you aware of it's existence.
Do you know your dogs generational history?
If there's a pattern you need a proper qualified reliable behaviourist. Spaniels, are eknown for resource guarding and ot does need dealing with properly.