it is common amongst behaviourists and trainers to refer to whatever it is that causes an individual animal (this applies to people too!) to react as ‘triggers’. Different stimuli can trigger a fearful, frustrated or excited response depending upon the dog and the situation.
If a single ‘trigger’ occurs, provided it isn’t too intense (close, loud etc) then despite being aware of it, the individual is likely to be able to cope and after a temporary increase in arousal their nervous system will return to a rest state. Of course, the more intense the ‘trigger’, then the more extreme the response and the slower the recovery time is likely to be. If the animal reacts to an something additional before their arousal level returns to the resting state, then they will be more likely to react to this new ‘trigger’ and so on infinitum! This state of affairs is often called ‘Trigger Stacking’
The veterinary surgery is an obvious example for trigger stacking to occur. Many dogs are frightened of the vets as a result of a painful condition causing a negative association, if the individual is also frightened of strangers as a result of poor socialisation then a veterinary examination which is already inherently frightening becomes even more so. Add to this a dog that is also worried by other dogs, and their experience of the waiting room will probably already have them is a state of high arousal. For certain individuals this may be enough to tip them over into an aggressive response. Of course, the veterinary surgery is not the only place where trigger stacking is likely to occur and advising on the management of different ‘triggers’ is certainly one of the things that a qualified behaviourist will attempt to communicate with their clients when dealing problem behaviours.
Hill D 2012 trigger stacking and stress hormones on-line video https://www.youtube.com/watch?v=IFGIRPAWcSM