I'm no expert, but I can take a stab at it.
What part of the brain is involved in this phenomenon
The amygdala becomes active and locks up other parts of the brain (thank you @KeeganKeplinger for helping me clarify). This is based on the fact that the amygdala is responsible for more responses than fight or flight, as I had previously thought. Recently I read a book by Missy Vineyard on the Alexander Technique, and this review of How You Stand, How You Move, How You Live sums up the relevant part I was looking for.
the four expressions of fear produced by the amygdala in the brain – attack, withdraw, freeze, and submit
is it a stand alone issue or is it always accompanied by some other type of social anxiety
This would be coupled with more facets of anxiety than just social. This report might also corroborate this. I know if I'm withdrawn, determined on fleeing or any other variety of emotional responses - my ability to speak, reason or do much of anything is fairly impaired.
Studies have also found that the amygdala modulates the fear
response in humans. Fearful stimuli including fearful faces,
fear-inducing images, and fear conditioned cues, have been
found to activate amygdala in several brain imaging studies using
positron emission tomography (PET) and functional magnetic
why do some men have it and others do not, is this some sort of protection or evolutionary byproduct and are there any studies specific to this issue?
Same reason as some people are easier to fright, others are easier to fright, I'd imagine. Life differences, not just past but present as well. Lack of experience approaching women (or men), and I don't believe this is male-specific. I dated a gal a few years ago who was the shyest person I ever knew. Over the phone it was easy to talk with her and get her to talk, in person she froze for the longest time. Was both amusing and frustrating simultaneously.
If one believes that the amygdala is the cause, then it'd be a matter of evolutionary byproduct. Freezing is a defense that's more passive than running or fighting, we try and protect ourselves by locking up and becoming like a wall. That book I linked covers the amygdala portion of the brain and it's role in our evolutionary history, as well as it's role in our current lives, as well as select other parts. The author seems well-versed in neuroscience, not just the Alexander Technique.