i think there's a security issue about knowing what you are saying is right. like the kids in school who are the best students but still get extremely nervous when they teacher calls on them. the idea of being wrong or making a mistake or not being capable is so terrifying that it locks you up and you just wait, postpone, defer, etc.
then theres the other issue of perceiving any difference as being embarrassing, shameful, etc. the tendency to imagine others are perfect and you are defective, you have something to hide, you are broken and flawed. i still have issues reading into any difference between myself and others as a potential weakness of my own. which wouldn't be so bad if you didn't actually expect that you should be completely perfect in every way. that pressure creates chaos internally, hesitancy, neuroticism, etc.
i see these as being rooted in the 5, 6, 7 head thought anxiety, the 2, 3, 4 heart feelings shame, and the 8, 9, 1 ____ ____ anger triad of the enneagram.
5w4 and 4w5 break down from the perfectionism that results with tying self-focus and knowledge/truth to the situation. you want to know the truth, but you want to feel perfect. and they can spiral.
a larger and more glaringly obvious problem that affects all N types is that sometimes they grow up and don't have any solid relationships bc they don't know other N types or have an N supportive environment. most of my entps say they just felt like a crazy person growing up, whereas enfps can find a niche easier, ESPECIALLY females, bc socially they're so skilled and they can be queen of any social group (including the geeks- if they decide to go that route).
entps have the really obvious 3w4 type who is more aggressively conventional when going after social achievements, the 7w6 who is the most feely and spiritual, and the 7w8 who is focused on going after what it wants balls-to-the-wall and just looks for the most engaging challenges.
ij types also have the added pressure of feeling easily overwhelmed. their judgment needs structure and objective for it to work properly, and if they are just totally out there with no guidance and social pressures and have to make quick improvised decisions without time to sort out the information they can easily hyperventilate. 9 types are shy bc they are so focused on others that they lose themselves and feel rudderless. (blackcat, how your wing would have kicked in and solved your problem, it is a part of your larger psychological structure so your shyness just evaporates. i think isfp 9w8 is more in touch with in-charge style Te than the 9w1 types whose Te is more repressed and inwardly critiquing/self-flagellating).
also, i find the intp perspective on shyness adorable. it makes me think of yoshimi (a robot learns to feel)! but recognizing all the while that what they say is 100% logical and probably the most intelligent thing that could possibly be said about the situation. most direct, specific, accurate weighing of the variables. yet feeling is such a gigantic obstacle, and intps are only shy in the first place bc they lack the internal structure to organize themselves. they need a game to bring out their best. when they learn the tools to play the social game they become very very capable of adjusting to any situation. they have such a wide range when it comes to interaction styles.