I think people feel comfortable in settings where they draw on a concrete body of knowledge (like most normal job interviews). With programming though, everything is logic, being able to dynamically model rules and patterns in your head is everything. You build these models in your head, using your programming talent, but they are like volatile memory, and you always need to reconstruct them. So when you go into an interview, you have little concrete knowledge to lean on, so off the bat you lack confidence and get worked up because you don't realise why this should be.