Without knowing the context of any particular case, I can only offer general comments:
The same question can often be asked of many religious people about why they are defensive about their religions. This is not a question that can be limited to Catholics.
People usually become defensive about religion for several reasons: they may lack the intellectual formation in their faith to answer the questions posed; they may become emotionally invested in the conversation and unable to discuss the issues dispassionately; they may feel that the questions are insincere or have been posed in an obnoxious manner, and are in fact an attack on something they hold dear.
When you are in a conversation with someone who is losing his emotional composure over the topic, it is usually best to call a halt to the discussion and change the topic. You might say, “Why don’t we table this for now until we both have a chance to do some more reading on the issues involved?” That will give each side a breather and a chance to re-evaluate his arguments without fear of losing face in front of his opponent.
God Help Me! These People Are Driving Me Nuts** by Gregory K. Popcak