People always imitate my actions on purpose. Are they laughing out of social psychology?

It may be that your movements are different or more noticeable, and imitating your movements can bring fun to everyone;

There is also that you are easy-going, people like to tease you, and you are popular.

As for sarcasm, it depends on your reaction. If you are cheerful, people will naturally stop laughing. If you resist and others imitate you like this, it is disrespectful.