The truth is that people often do avoid grievers. Since our society has so mis-educated us about loss, we are often led to believe that the griever wants and needs to be alone. Although grievers sometimes want to be alone, more often they want to be treated normally. Because we were never properly taught how to talk about the conflicting feelings caused by loss, we are often afraid to talk to our friends when they have experienced a loss. Therefore our own fear will cause us to avoid grievers or to avoid the subject of their loss.
Fear is one of the most common responses to loss.
Look at the combination outlined above. People avoid grievers because they are mis-informed and afraid. Grievers avoid others because they are afraid and then isolate. That does not mean that every griever will want to have a detailed conversation with every one they meet. Nor does it mean that you always have to make yourself available to someone who may need more time than you have.