My personal thought is while there is a lack of boundaries, there is also a threshold of confidence being exceeded in these 4ish circumstances.
Young horses will push boundaries, that is a known fact. Rather it is with the dam, friends, or human, young horses are going to test the waters as they start coming into themselves. I’ve worked with hundreds of foals (thank you TB breeding farms ) and I usually see a lot of sass begin at 2 months, reappear around 6 months, and really start solidifying by 12 months. Early correction and detection is essential, and if caught early, usually as a yearling they have been sorted out. This could mean 1 of 2 things. Either her antics were found funny when she was younger but has now reared its ugly head (no pun intended) to an extent that is no longer funny, or something medically has occured that may cause random symptoms (i.e. ovarian related).
My thought leans to the idea that this was not corrected directly, and then leads into her confidence level not being as high as another horse’s would be. They are all different after all. My suggestion is to start introducing more mental stimulation that requires that threshold to be met, and safely manageable if it gets unruly. We never know where our horses lines are drawn until we start hunting for them. For example, my horse is quiet and calm on trails. You can pack him, lead him, have a string, whatever you want, but you bring out a flag…all bets are off. He loses his mind. So I say, “ok, there’s that line, now let’s see what I can do to maximize the threshold.” Suddenly, his confidence increases and I have less blow ups in general. I will correct dangerous behavior but I will never correct spooky behavior. I learned that if I allow the horse to be a horse, which is spooking, then they find out that I don’t care that you’re worried, I just care if you try to kill me in the process. So now I’m seeing a horse with a little thought behind the eyes versus full blown panic.
This is simply what as worked for me and is by no means backed by science or top trainers. But I like the conversations with my horse that I have now. He now turns to me when he’s concerned and then thinks on things, rather than flee the scene. I also put myself in a round pen and my horse naturally does NOT want to drag you down the road. So I have 2 things that work in my favor to help create a horse that is more willing to work rather than peace all the way out.