The usage of the term suggests that the person has taken up an unassailable position, and that he or she is not open to discussion.
Perhaps some are this way, but certainly not all atheists. I know in my case I am open to discussion. I will hear what others feel are evidence for their deity. Also my position is open to change if objective evidence is presented.
What then follows is an affirmation of your suspicions as to the rigid mindset, with a flippant, emotional snarl "Why should I have to prove X? The default position is X."
I understand what you're saying here. It is certainly the case that some atheists or theists are "touchy" and may "snarl" when confronted about their beliefs. But the response is not unreasonable. "Extraordinary claims require extraordinary evidence." The default position should be disbelief until objective evidence is presented. This approach in itself does not make someone closed-minded.
But yes, we can find intellectually lazy people on any side of an issue.