The Straw That Broke The Camel's Back
As I was placing the little statues of shepherds and sheep, kings and camels around our Christmas manger, my mind wandered to the saying "the straw that broke the camel's back" and it suddenly occurred to me that the holidays can really heap a lot of "straw" on our dogs. "Straw" being triggers,
things that increase your dog’s stress level, which cause a reaction. If the stress level is fairly low, the reaction may not be dramatic or even noticeable to many people. But if the stress level is high, the dog’s reaction will be much more obvious and could potentially be dangerous.
Different triggers produce different levels of stress. A low-level trigger, such as a pat on the head for some dogs, would produce low-level stress and a very high-level trigger, such as fireworks, would produce a very high level of stress. Triggers can be cumulative, meaning that a bunch of low-level triggers can have the same effect, or sometimes even more of an effect, as one high-level trigger. Adding one trigger on top of another, or combining triggers, is called trigger-stacking and this negatively affects the dog’s ability to cope with the stress she is feeling. Trigger-stacking can happen all at once, or over a course of time since the chemicals released during stress take quite awhile to go back to normal levels.

Here is one example of trigger-stacking: Perhaps your dog is mildly afraid of children and very afraid of loud noises. One quiet child in the room might make your dog mildly nervous. Two or three quiet children would heighten that stress level and you might see the dog looking away, showing the whites of the eyes, yawning, or exhibiting other calming behaviors which help her to cope. Now suppose you change these to one or more
noisy, boisterous children. What do you suppose would happen to the dog’s stress level and ability to cope? Adding noise + children could easily send her stress level over the top and she might try to lunge or bite. The combined stress of noise and children is kind of like that last straw, the one that broke the camel's back.

Claire Staines of Lothlorien Dog Services in the UK gives a nice visual description of how trigger-stacking works in this video:
https://youtu.be/SzEkeZ0E4kI
Other things can add to your dog’s stress, too. Changes in routine, for example, which happen during the busy holiday season. The dog may be fed earlier or later than usual, their customary walk is shortened or omitted from time to time, you come home later than normal because you stopped to shop, etc. Changes in the environment can also add stress: A big tree in the middle of the living room, fuzzy snowman decorations that the dog gets scolded for playing with, blinking lights, etc. All of these changes happening over the course of a few days may seem insignificant, until the last "straw" is piled on and the dog snaps at the toddling grandchild who simply squealed in delight over a toy.
You can see how the holidays, with all that make them so special to you, could wreak havoc on your dog's coping skills.
This holiday season, keep your dog's comfort in mind when you invite guests for parties or dinner. Keep everyone safe by knowing what the triggers are for your dog and avoid trigger-stacking by letting her hang out in a quiet room, crated if she likes being crated. If your dog is a social butterfly, by all means allow her to mingle with the company, but watch her body language for any signs that she might be getting stressed and less able to cope with the excitement. And always supervise your dog's interaction with children. Even the friendliest of dogs has a limit. If you have a multi-dog household, remember that what may be innocuous for some dogs may be a trigger for another. Treat each of your dogs as an individual and separate them if one's stress seems to be having a contagious effect on the others.

May your days be merry and bright, and may we all have a peaceful holiday season!!