There are a three major reasons why scientists think the breast milk of far northern peoples is so toxic. First the toxic chemicals, which originate in warmer southern climates, are persistent. They stay around a long time. The main ones are flame retardant chemicals and also PCBs (PolyChlorinated Biphenyl) but there are other so called Persistent Organic Pollutants. These are very long-lasting man-made compounds that do a good job where they are used, for instance to protect furniture and carpets from catching fire, but they do not degrade naturally very fast, so they tend to remain in the environment for a very long time.
Secondly, it seems that these toxins tend to migrate through the atmosphere to colder climates. This is probably because in warmer southern regions where they are used the most, a small but steady amount is continually evaporating into the atmosphere. These toxic chemicals last a very long time so they can move around in the atmosphere for years. Eventually some makes its way to the North pole, where it's very cold, and in these conditions a certain percentage of these pollutants condenses out of the atmosphere and finds its way into the food chain, probably first being incorporated into very small plants such as algae, which are then eaten by larger and larger animals.
This brings us to the third part of the problem. These types of toxins are almost always soluble in fats, so as they get into the fat of small animals, which get eaten by larger animals, then larger ones, the toxins get increasingly concentrated in the fat of these animals. In the North, Inuit people love blubber and have a tradition of eating and enjoying the fat of seals, whales, fish and other arctic animals. They eat a lot of fat mainly as a survival mechanism for very cold climates. And the Inuit are at the top of the food chain, hence the toxins are most concentrated in humans, both men and women, but it's the women that produce breast milk, which also contains a lot of fat to help the baby grow.