He never really had issues like this before, but this year he has been competing inredibly hard and has been jumping higher and getting ready to move up to Intermediate level eventing. The ground where we're located can become quite hard, especially after dry summers, and only a few days before he started displaying foot-soreness he spent a weekend schooling a lot of cross country fences of significant height on this hard ground. I don't know if his hooves were packed afterwards with something like Magic Cushion or Rebound. Usually he doesn't get his feet packed so I don't think he got any of that.
The horse started being shod by a new farrier in May along with the owners' 4 other horses and my mare. None of the other horses have had any issues with their feet (minus my mare, but she had a lot of problems when I got her and her feet have only been getting better and better). So, back to my original question; can a farrier cause a horse that previously seemed to have no problems with his soles to become more thin soled? I don't know what the thickness of his soles was when they got him (I'm sure they had his hooves x-rayed, he was a pretty expensive horse and they covered all their bases when it came to pre-purchase exam, as far as I know) but apparently since being on Farrier's Formula he's grown about 5mm.