It can be learned.
Someone who has been socially rejected due to no fault of their own, or who has received pain from people for whatever reason, over time will be conditioned to believe people equal pain, thus it will result in them avoiding said pain. Meaning "introversion".
Someone who has been socially accepted by praise, friendships, etc, will be conditioned to believe people/groups equates to pleasure, thus they'll learn to seek the pleasure.
We tend to seek that which gives pleasure, and avoid that which brings pain. If introversion helps one avoid the pain of rejection, abuse, whatever, then of course they'll be introverted. Which means introversion may not be all natural.