Meat – Cooked meat diaper: toxic

Per the USDA guidelines, frozen chicken should not be cooked in a slow-cooker or a microwave. It can only safely be cooked in the oven or on the stovetop.

A similar warning is given for beef as well.

To quote a comment:

According to Harold McGee's "On Food and Cooking", when meat is cooked for an extended time below 120˚F or brought up to temperature slowly the myoglobin remains intact and there will be a distinct red color throughout the meat. – Didgeridrew Sep 23 '13 at 16:39

Here is the Science of Cooking article @Digdgeridrew mentioned

To quote from there:

When dark meat is cooked, myoglobin's color changes depending on what the meat's interior temperature is. Rare beef is cooked to 140° F, and myoglobin's red color remains unchanged. Above 140° F, myoglobin loses its ability to bind oxygen, and the iron atom at the center of its molecular structure loses an electron. This process forms a tan-colored compound called hemichrome, which gives medium-done meat its color. When the interior of the meat reaches 170° F, hemichrome levels rise, and the myoglobin becomes metmyoglobin, which gives well-done meat its brown-gray shade.

Now to address why it was on the outside I'm going to make an educated guess. Per Wikipedia:

Myoglobin is an iron- and oxygen-binding protein found in the muscle tissue of vertebrates in general and in almost all mammals. It is related to hemoglobin, which is the iron- and oxygen-binding protein in blood, specifically in the red blood cells.

I have personally experienced this same phenomenon slow-cooking stews, roasts and even chili in the crock pot. All perfectly normal. Have you noticed when you cook red meat over high heat, like grilling a steak for example, the blood remains inside the muscle? I have noticed slow-cooking tends to "release" more of the blood content from inside the meat, likely due to more even cooking. This in my view would explain also why myoglobin would be found on the outside, rather than inside, of the meat.