Really upset about this. Could thus be genetic, or just a fluke?

A lot of foals are born with restricted tendons or tendons that aren't tight enough, and there are some things you can do to help the tendons tighten or loosen, it just seems like sometimes it is luck of the draw. It also isn't necessarily true that the foal would have had to be put down, because the problem may have been corrected with boots, etc., and it is certainly just bad luck the foal was breeched (this is why the foal died in the first place- he couldn't be expected to survive a birth like this). However, I do have to agree with your hesitation in breeding your mare again. She may have damage internally that you don't know about from the foals breech, and you were lucky that you didn't lose your mare as well. A breeder I used to work for almost lost a mare to a breeched foal- a special mobile operation team had to come and surgically remove the foal, and to do so the foal had to be chopped into pieces. The mare was never bred again, because the chances of that happening again were increased, and the damage to her too great. Sometimes, after a breech, if you wait a few years, the scar tissue and everything heals on its own and the mare can be safely bred, but I would definitely get ultrasounds and everything done before you do so, to check everything out.