It's Monday. Your alarm didn't go off this morning and you're running 45 minutes late for work. After rushing through morning traffic, you spill your coffee on your white work shirt getting out of your car. You have to walk into the office like this and your boss chews you out in front of all your co-workers. You deal with a big, stressful load of paperwork and grumpy customers all day. You get cut off in traffic twice on your way home. At the end of your day, you walk through your door and your kids are screaming over a toy. The dog is barking and jumping at a squirrel he sees through a window.
Your significant other casually asks you if you can do the dishes after dinner.
It's the straw that breaks the camel's back. You lose it and start yelling at everyone in the house.
This is an example of trigger stacking causing over-threshold reactions.
Trigger stacking occurs when multiple stressful events happen simultaneously or within a short amount of time. Cortisol, adrenaline, and norepinephrine, which are stress hormones produced by the adrenal glands, build in the system. This will lead to responses that stem from the sensation of being deeply overwhelmed and trigger a reaction that is out of measure for the singular stimulus (a.k.a "amygdala hijack") an organism is experiencing in their "now."
What if only one of those things in the above example happened over the course of a week? Would that still be stressful? Totally. Would it be as stressful as all those events happening within 8 hours of a single day? Probably not. You were over-threshold. When you lost your temper over having to do the dishes, your amygdala was hijacking your brain.
Dog can experience this just like we do. That phone call usually sounds like this: "He/She has never done (blank) before. It came out of nowhere!"
.... has never snapped at me over his food bowl, never lunged at another dog, never picked a fight with our other pet, never growled at my kid, never attacked my neighbor, etc.
Your dog was likely trigger stacked.
Your dog woke up to the gardener using the leaf blower outside, she saw three squirrels on her walk, a loud motorcycle passed by her too closely on the street, she didn't get her usual nap when you got back because the kids wanted to play, the cat smacked her in the face. This all happened before three in the afternoon.
You reach for the toy next to her and growls, lifts her lip, and flashes her teeth at you.
The second half of understanding trigger stacking includes an imaginary ladder or picturing rising water levels.
Some dogs will "climb the ladder" to an over-threshold response much faster than other dogs will. Dogs with a history of anxiety, reactivity, and ignored or unnoticed signs of discomfort are likely to "skip steps" on the ladder and "go under" water levels to reach the point of reaction much faster.
Here's a great simplified graphic on it:
A Dog's Stress Level is Analogous to Rising Water
Our goal as dog owners is to keep our dog's heads above water. If your dog is struggling with over-threshold reactions and you're having trouble discerning what their triggers are, or if it seems like they climb the ladder so quickly that there are none, try bringing a qualified force-free professional in to help you assess the case.