1. Separate growth of shell and meat: The growth process of oyster is separated from shell and meat, which makes it better adapt to the environment during the growth process. During the growth of oysters, the meat will continue to grow while the shell will remain relatively stable. This separation is natural and usually does not have any adverse effects on oysters.

2. External stimuli: If oysters are stimulated by external stimuli, such as temperature change, water quality change or environmental pollution, it may lead to the separation of shell and meat. These stimuli may cause the stress response of oysters, leading to the separation of shell and meat.

3. Diseases: When oysters are infected with certain diseases, it may also lead to the separation of shell and meat. For example, oysters infected with viruses or fungi may lead to unbalanced growth of shell meat, which may lead to separation of shell meat.

4. Over-cooked: If the oyster is over-cooked, it may also lead to the separation of shell and meat. Overripe oysters may cause shell-meat separation due to unbalanced growth of shell-meat.

No matter what causes the separation of shell and meat, it does not necessarily mean that oysters are dead or stale. If it is found that oyster shell meat is seriously separated or accompanied by other abnormal conditions, it is recommended not to eat it. If you are not sure whether oysters are safe to eat, you'd better consult a professional.