Reading the Signals Before the Buck
Horses very rarely explode without warning. They escalate, in a fairly consistent order, and most of what looks sudden is the last rung of a ladder nobody was watching.
"It came out of nowhere" is one of the most common sentences in horse ownership, and it is almost never accurate. What is usually true is that the earlier signals were quieter than the final one, and nobody was reading them.
Horses are not built to escalate straight to violence. It is expensive and dangerous for them too. They work up to it, and the sequence is reasonably consistent.
The ladder
At the bottom are the signals so small they are easy to dismiss: an ear turning back, a tail swishing when nothing is biting, a slight tension through the body, a horse that has stopped chewing.
Above that, the horse starts to change its shape. The head comes up. The back hollows. The stride shortens. The horse feels tight rather than swinging.
Above that, it begins to remove itself. Slowing, stopping, drifting sideways, refusing to go forward, turning back towards the gate.
Then the larger objections: head tossing, running backwards, rearing, bucking.
The last item on that list is the one everybody notices, and it is the only one that gets described as coming out of nowhere.
Bucking is not the beginning of the conversation. It is the horse's last remark in a conversation that has been going on for a while.
Why the early rungs get missed
Partly because they are subtle, and partly because a lot of them look like nothing much.
A tail swish is easy to attribute to a fly. An ear back is easy to attribute to listening. A shortened stride is easy to attribute to the surface. Each individual signal has a harmless explanation available, and taking the harmless explanation is comfortable.
The tell is not the individual signal. It is several arriving together, or the same one arriving repeatedly in the same context, or a pattern that is new for that horse.
What happens if you punish the early signals
This is the part that matters most, and it is where genuine danger gets created.
A horse that swishes its tail and gets told off for it, or pins its ears and is corrected, learns something specific: that the early rungs are not worth using. The discomfort that caused them has not gone anywhere.
What you end up with is a horse that has been taught to skip the warnings, and skipping the warnings is precisely what produces the animals that genuinely do seem to explode without notice. They are not unpredictable. They have been trained out of being predictable.
The two questions worth asking
When the early signals appear, there are only two things worth ruling out, in order.
Is it physical. Pain and discomfort produce every single item on that ladder, and a horse that has recently started objecting to something it used to accept is a veterinary question before it is a training question. Saddle fit, teeth and back soreness account for an enormous proportion of what gets labelled as attitude.
Is it comprehension. A horse that does not understand what is being asked, or that is being asked for something it is not physically ready to do, escalates in exactly the same way as one that is uncomfortable.
Neither of those is solved by pushing through, and pushing through is what most people do, because the signals were small enough to look like something to ride past.