I think the common ones fall into a 'fear of losing social status' bucket.  Looking dumb, or deviant, or otherwise being perceived as a poor social investment for others to put energy into.

The actual word "insecurity" implies a fear of loss, which fits this model.

I'm a bit unsure what you mean by social insecurity, but I'm going to take it as something like social anxiety that people feel.

Most social anxiety, as I see it, comes from a fear of exclusion rooted in the evolutionary very real threat of death as a result of exile. Today few people actually die of ostracism, but it certainly makes one's life worse, and social reality includes a lot of in between where you might not be at risk of dying but certainly you'll lose status and access to opportunities if you look bad. From there we worry about any number of things.

Some things that seem common to me:

• offending a high status person
• looking foolish
• thinking things are fine but people are talking behind your back
• imposter syndrome (not cool/smart/etc. enough to hang)

I guess most people are afraid to do anything that is unusual in their social circles. That may include even things like reading books... if your friends don't read and think that reading is low-status. Even having an unusual skill can be seen as low-status.

Sometimes this is not internalized, e.g. the person may still enjoy reading a book if they know that their friends are not around to see them. Sometimes it is, e.g. some people would not do something their parents disapprove of, even when they are alone. (Maybe with parents we have a strong feeling that sooner or later they would find out? Or maybe parents are more successful at convincing us that the society at large shares their opinions?)

Hmm., let’s see:

Fear of losing status, writ large, is an obvious one: fear of not being able to keep up with one’s friends and relations, of not being good enough, etc.

Fear of gaining undeserved status / impostor syndrome? Which, I would guess, includes what high-agency people usually refer to when they talk about "fear of trying anything one isn’t used to doing"?

Fear of not knowing where one should go? That one I’m less sure about, since I mean mostly things like "not being sure one has made the right choices in life/midlife crisis/young folks who don’t know yet where they are in the pecking order". Maybe it’s a vague umbrella over stuff that should be in the other two categories, maybe it’s not as clearly "social" and more "fear of not living the life one imagined for oneself"?

Not being liked, being thought of as other or outside someone’s social purview.