There are different explanations.
One theory is that animal proteins decrease the body’s life energy level to the point of toxemia.  The culprits can include neurotoxins produced by proteolytic bacteria, such as Clostridium, or by yeasts, such as Candida. Yeasts can develop rapidly when the daily diet contains both animal proteins and complex carbohydrates. Meat can favour the development of these bacteria by blocking or delaying the passage of carbohydrates through the intestines. The neurotoxins increase the sensitivity to any external stimuli, such as the noise, the light, the sun, etc.
Another possible theory is that this proteolytic bacteria is the cause of the leaky gut, which is the main door to hypersensitivity, by allowing the passage into the blood of substances that shouldn’t be there.
But the most plausible explanation is based on the fact that bacteria that feed on animal proteins releases an important amount of histamine. Sun exposure releases histamine, but if the body is overwhelmed breaking down the huge amount of histamine produced by meat and fish, then it can not deal with the normal histamine coming from other sources, such as the sun.