I've seen well trained, respectful horses, that would hard tie, behave under saddle and on ground, who never backed an ear, or made a threatening gesture, turn into completely dangerous animals in just that little a time frame.
An owner/handler who allows horse to get away with murder, who tiptoes around them and overlooks any sign of aggression from the horse.
I've seen a mare that I handled every day, that was NO trouble at all, could hard tie her for hours, and she would go to sleep, could groom her, pick up her feet, tack her all without any problem. This mare was 8 and shown all her life basically.
Goes to owner's barn, where handlers are A) into the "oh you are so sweet" mindset, or B) totally new with horses and into the kissy kissy feed treats game.
In one week mare had turned into a complete idiot that was fast becoming dangerous.
Actually sat back on crossties, and flipped herself because was being tacked up.
Every horse in that barn acted that way and it was for no other reason than horses were spoiled and not made to mind.
Every horse that I have been around that has had problems? 99% of them are because the humans around them either had no idea what they were doing, got mind melded into "oh my horsey loves me" set, or ignored the common sense of others around them.
In some cases, horses ended up going down the road because the owner had ruined them to point of no return, and in others, horses ended up seriously injuring owners, or killing them, and ended up paying the price.
Horses can kill you in a Georgia heartbeat. They will not think "Oh, I shouldn't have done that" as much as "I showed them!"
And nothing more aggravating to sensible horse people than to watch someone continually doing something stupid with a horse, who then asks for advice because of problems they are having, and who then proceeds to tell you why your advice is wrong.
I am not going to play games with horsey, nor am I going to stand and try to read mind as to whether or not little horsey feels like working that day. And I am not going to put up with any attitude at all from a horse.
They aren't dogs, don't care if you show up or not, as long as there is feed, and would just as soon run you down as look at you if they know they can.