My sympathy, because I can imagine this must be puzzling.
From experience, my analysis tree would go like this:
1. Is this a euphemism or excuse? Euphemisms can be used to cover up other meanings like "you look like a serial killer" or "bathe more." You might want to pursue those angles. Excuses can cover things like you wanting too much money, or being too strong in your opinions or knowledge. (Remember, the machine wants cogs, not governors.)
2. If you think it's actually what they mean, I'd look at the word as they're going to use it, which means "socially withdrawn" and not its actual meaning. Then I'd check these:
(a) Does my appearance scream "socialization difficulty"? Did I wear a monk's habit to the job interview, or talk about 1980s classic videogames too much?
(b) Does my behavior scream "socialization difficulty"? If so, I'd approach this as you would any other technical problem. Find some "normals" to observe and pretend you're James Bond, super spy, and imitate them. You must infiltrate the hive and shut off the reactor.
3. If none of the above are checked: something else is making them nervous. Talk to a recruiter, psychologist or other and have them observe you. Also, make sure no parts of your life or resume scream "serial killer" or "probably molests goats."
4. If that's not the case, think about where you're applying. The machine loves cogs -- I said that already -- but sometimes, silly people in charge of silly companies want cheerful happy cheerleader types who will socialize a lot, get really excited over foosball, etc. They like these because they're tractable and easily manipulated. These silly companies tend to be tech sweatshops that assess you based on the amount of code you grind, not how good it is. Do you want to work there anyway? If you do, you're going to have to play Halloween dress-up and imitate what they want.
I will never support bias against introverted people just so that there can be the illusion (sorry, I mean "appearance") of a happy enthusiastic go-get-'em workplace. That's just silly. But if your appearance or behavior makes other people nervous, and they're not nitwits, you might consider adopting a little bit of a charade so you can work with these people.