Guys can feel intimidated if you radiate lots of power, confidence or assertiveness.
Being rejected by a girl is usually not something they enjoy.
A model on a catwalk for instance is intimidating because there is this high sense of confidence, no smile, gorgeous looks, etc.
If you wear lots of black or look like a top executive, it can be intimidating as well.
If you want to be more approachable, you can soften the way you dress, smile often, establish eye contact and develop a friendly attitude to anyone you meet.
Simply show that you are open to meeting new people and that you won't reject anyone who approaches you.