I think you will find its the way its been killed and bled, with out going into to much detail, if the animal is stressed at the point of slaughter then the body and muscle tense, causing adrenalin to rush around the body tainting the meat.
Also if the animal has not been butchered and bleed properly, this will also taint the meat, especially if during butchering some of the organs have been punctured.
Of course you are not going to know this if you have not been present during the slaughter, as once the meet has been washed off it will all look the same, until of course you come to cook it then try it.
And if the animal is old.
Hope this helps
Darrell