Why marijuana triggers panic attacks for certain people?

Here's my guess: some people's anxiety is a defense mechanism for their body. It allows them to be constantly vigilant about things they perceive as threats to their physical and emotional well-being. Pot can be very physically relaxing and mentally distracting, leaving highly anxious people to feel less vigilant and more vulnerable. Their body has a panic response to this, because on some level they feel like they're in danger.