This horse has had hocks injected in February and has been firing great.  Saw the vet three weeks ago, had poll injectinos and a little chiro.  Sunday we were at an outdoor arena where the ground is kind of hard.  I wouldn't say it is bad, just different.  I was really pushing him and after he rounded the 2nd he just blew up like a bronc.  We both took on air and i hit the dirt hard.  Horse was fine all day, was fine in the alley and fine going in.  Also fine when caught at the gate and i got back on him for the walk to the trailer.  I was too sore and could hardly catch my breath or I would have rode him longer.  I have had this horse for 4 years and this is not his normal behavior but he also is the first to safety up on ground that isn't like fluffy deep indoor.    Do you think he just didn't like the ground and flipped me the bird or something else going on?  

Sometimes they just have a attitude 

If he needs injected, that means there's damage. Which is probbaly why he safetys up on ground he doesn't like. So its possible he just hit something the right way on some different ground in the turn that sent a pain wave through or it just flat hurt & that was his response.

Pole soreness is usually a sign of soreness in the hind end. Bucking as you described sounds like stifle soreness. 

Have you had his back x-rayed? A lot of issues like hocks and stifles can be secondary to something going on in the spine. It's pretty cheap and worth it to rule out anything going on back wise. Good luck! 

My mare, who is not a bucker AT ALL, almost bucked my daughter off when we were riding together. I knew this was totally out of character for her so she immediately went to the vet. She had a small tear in her stifle. My vet said in her experience, stifles and bucking correlate with each other. Good luck, I hope you get some answers.