It's actually a result of freezing the meat during transport. The juice is a mix of water and myoglobin. Myoglobin is a protein within the muscle. Most meat is made of 5% fat/carbs/minerals ...

But this couldn't be further from the truth, and it has a lot to do with a little protein called myoglobin that's responsible ...

Cardiac enzymes ― also known as cardiac biomarkers ― include myoglobin, troponin and creatine kinase. Historically, lactate dehydrogenase, or LDH, was also used but is non-specific.