What makes some hemoglobinopathies unstable?

Trait vs disease. Hemoglobinopathies are a variety of inherited diseases where the hemoglobin molecule (part of RBC that attaches to o2 which all cells need to live) is malformed. Examples are sickle cell & thalassemia anemia. Abnormal gene from one parent, usually have 'trait' and live healthy life. Gene from each parent, get the disease: infection, stress, a cold, other events can set off a crisis and instability.
Hemoglobinopathies. When the structure of hemoglobin is altered due to mutation then it is more prone to earlier destruction by different mechanism. As a result the red blood cells will age and destroy much quicker than a red cell with normal hemoglobin which as a lifespan of 120 days.It is diagnosted in early age in infant due to anemia related complications, and hemoglobin byproducts metabolism.Hope it helps.