If you'd been hypnotized in a nightclub act to do something silly afterward at the hypnotist's command, would you then ask, "Why am I clucking like a chicken?" No. You've volunteered to let someone make strong suggestions about your behavior.
As a child you were even more suggestible. Not only did you learn what was expected, you also learned how to maintain that trance. As an adult, your conscious mind and unconscious programming work together to keep the suggestions operating. Check it out for yourself. How many times a day do you say to yourself, "Don't be rude," or "Look out for yourself, nobody else is going to," or "_______________" (fill in the blank for yourself).
Does this mean you have to like the programming? No. You may consciously feel the urge to change, or you may have headaches, or tense shoulders, or acid indigestion, or depression. Something tells you this isn't who you want to be. These are wake-up calls -- your attempt to snap your fingers and break the trance.
You could spend the next few years exploring why you have the patterns you have. Or, you could simply accept that you've been in a trance and ask how? How does my trance operate? What triggers it? When you observe this closely, you'll know how to break the pattern.
It's not a surprise that we associate "being chicken" with powerlessness. But notice the implied meaning of "free-range chicken." You, too, can range free.