I have always been an avid 'observer' of horses and training techniques. I have done this since I was a child. Observation and studying 'cause and effect' were my interest longer than I can remember. This has included watching herd dynamics in herds of all mares, all geldings, mixed herds and 'natural' herds with at least one stallion. They are all a little different but are all quite a bit the same. Then, seeing how those different herd members 'trained' and interacted with people and accepted and learned from training techniques has very much fascinated me for most of my 66 years.
It has been my observation that bucking is a 'natural' athletic move made by a normal horse as a fear response. A feral horse or an untrained horse that has not been taught to accept a saddle or something around the girth / belly will naturally try to buck off a saddle if it is just introduced without any preparation and desensitization to the unnatural feel of the saddle and girth. To an untrained horse, it is no different than the response you would observe if a mountain lion jumped on the horse's back.
When I watched bronc riders put a saddle on an untouched horse, it would fight and squeal and buck, sometimes throw itself over violently, would sometimes try to attack the handlers and usually ended up tied down or snubbed and choked or nearly choked down.
If the horse was turned loose to 'get it out of his system', he would frequently buck to exhaustion or near exhaustion and finally quit. Some would buck with a rider then and some would not. Some would buck again the next day but others would not. Some bucked every morning for many days and some never gave up the new 'habit' created by throwing a saddle on and letting them buck. Some of these horses will buck in the morning 10 years later. Some got sick of bucking and really did get it out of their system and others just kept getting more practice and kept getting better at it. Some turned into 'cold backed' horses that bucked every morning or every morning when they were particularly 'fresh' or it was really cold or their rider got in a hurry to saddle and mount or ???? They were just looking for an excuse for years to come.
Bloodlines in the registered horses or those herds that had a registered stallion (as was common on early day big ranches) were pretty consistent as to whether they got over it or got better at it. I was not very old by the time I had gotten pretty good at predicting what different horses were going to do.
By the time 10 or 12 years old, I was determined that you could 'teach' a horse to accept a saddle just like you could teach him anything else you were good enough at to teach. By the time I was 13, I was accepting horses to 'break' and very few of them EVER bucked. By the time I was 20, I was starting 50 head or more every year and 1 or 2 would buck THAT SOMEONE HAD NOT ALREADY TRIED TO BREAK AND LET BUCK BEFORE I GOT THEM! These are the methods that I built on and I still use today.
I never, never, NEVER let a horse buck with a saddle, EVER. I gradually get a horse used to a girth by using a rope and then letting a horse stand around tied with surcingle on. When I first put a saddle on a horse, it usually does not do anything or even act like it has anything new on it.
If a horse is pretty 'goosey', jumpy, pushy or fearful, I may saddle him up in the morning and let him stand around all day just saddled. Every time I walk by, I will grab the saddle-horn and shake the saddle hard, flop the stirrups around and just keep doing this until the horse fully accepts the saddle and every move I make around them.
Everything I taught myself to do came from watching the bronc riders and cowboys that skipped steps 1 - 20 and went straight to step 21. I was so determined that it did not have to be that way that it completely molded what I did later.