It is the imposition onto other people that makes it antisocial. The "I don't care or won't consider how this impacts other people, I am going to do whatever I want or whatever is easiest for me. If it bothers, upsets, or imposes on others, I don't care."
To keep with the example below, walking in the woods alone is "not social" but it's not at all "anti-social." Listening to music with headphones while walking through the woods is not anti-social. Blasting music on a portable speaker, not caring whether or not you're disturbing other people on that trail, is a pretty great example of anti-social behavior. As is having a speaker phone conversation on mass transit, or being visibly under the influence of drugs in public, or choosing to park illegally and block someone in because you'll "just be a minute" or any number of other things people do because it makes their lives 5% easier at the risk of making someone else's 10% harder.
To keep with the example below, walking in the woods alone is "not social" but it's not at all "anti-social." Listening to music with headphones while walking through the woods is not anti-social. Blasting music on a portable speaker, not caring whether or not you're disturbing other people on that trail, is a pretty great example of anti-social behavior. As is having a speaker phone conversation on mass transit, or being visibly under the influence of drugs in public, or choosing to park illegally and block someone in because you'll "just be a minute" or any number of other things people do because it makes their lives 5% easier at the risk of making someone else's 10% harder.