If you speak about involuntarily hypnosis you need to define in what sense you believe that humans process free will. Without a concept of free will the concept of something being involuntarily has little meaning.
If you ask a person on the street about the way to the next supermarket, does that person go voluntarily or involuntarily into a trance? The person doesn't consciously decide to go into a trance. He however usually goes into a trance because that's what the social contract demands.
This puts the hypnotists into a position where give the subject a few suggestions. If he will give suggestions that the subjects understands to go against what situation is about the subject will snap out of the trance.
The hypnotist can, however, issue suggestions that change the person's perception of the situation without the person being aware that his perception gets changed. Computer hacking via privilege escalation is a good analogy. Whether a hacker can hack a computer depends on the specific software that the computer is running and the skills of the hacker.
What the hypnotist can achieve depends very much on the skill of the hypnotist, the suggestibility of the subject and the social contract of the situation.
Let's move to the chicken example. The core question is: "Why don't people act like chicken in real life?" There are psychological reasons that explain the behavior. There is a strong social pressure that labels it unacceptable to act like a chicken. In stage hypnosis the hypnotist sets up a situation where that pressure doesn't exist. He convinces the subject that the subject has no choice over whether to act like a chicken. The subject is more susceptible to commands while it's in the trance.
It's possible that the subject who wouldn't have thought before the performance that they would act like chicken actually acts like a chicken during the performance. If that's the standard for something being involuntary then it's possible to get people to act involuntarily.
If you use another concept of free will you might say that the stage hypnotist designs the experience in a way where the subject voluntary decides to follow the instructions.
Given the limited amount of time that a stage hypnotist has, the trance of the subjects usually isn't very deep and the stage hypnotist chooses a person who's more susceptible than average. A hypnotist who has a few hours with a subject can generally archive more than a stage hypnotist who has 5-10 minutes with a subject.
Let's take an example that isn't about stage hypnosis from the book "Monsters and Magic Sticks: There's no such thing as hypnosis?" by Steven Heller Ph.D. and Terry Steele:
While in this somnambulistic state, the subject was told that when a clock chimed 10:00 P.M. at that evening's faculty party, she would remove one of her shoes; place it on the dining room table and put roses into the shoe. Further, it was suggested that she would have no memory of the suggestion; it would appear to be her own idea, and she would feel compelled to finish her task. "A very interesting thing happened on the way to the forum." While she was carrying out the hypnotic suggestion, the professor asked her what she was doing. She replied that her husband had given her a beautiful crystal vase that looked just like her shoe and she had never known what to do with it. She went on to state that it had suddenly dawned on her how to arrange flowers in the vase and she had to try it in her shoe before she forgot.
Did the subject in that example act involuntarily? She attended the hypnosis session on her own "free will". She didn't act against her own intentions and was able to justify her actions afterwards.
The hypnotist still determined her actions. The common sense notion of something being involuntary breaks down when you analyze the example.