First, let's make sure you are not suffering from some pathology! There are situations where people do suffer spontaneous and uncontrolled movements due to a problem in their nervous system. These have been reported after strokes, for example. So, my first question would be - how is your health? Have you talked to a health care provider about this? Does it feel unsafe/scary to you? Can you take back control anytime you want? Movement disorders are often associated with Parkinson's Disease, autoimmune disorders and other problems. You may want to rule these out.
Assuming you are totally healthy, such an experience becomes more of a mystery. You are not alone! Many people feel similar releases. I have often had meditation students describe the same thing. What is actually happening? I can only speculate, as I know of no scientific studies that have investigated this, but there is a lot of yoga lore that talks to it. Unfortunately, my experience with such yoga lore is that it is more descriptive that explanatory. Yogis will talk about prana movements or even a Kundalini awakening, but as you suspect, I doubt these are causing what you have experienced. From what I have learned, Kundalini is like an atom bomb exploding, and if a yogi is not ready for this amount of energetic release, they can cause themselves grave physical and psychic damage. A prana movement is much milder and could be considered a cause of your movements, but does that help to know? What prana? How is it moving? Is it safe/harmful? How do you control it? These type of energetic questions are best answered by seeking an experience prana yoga master, in person. Such a person is unfortunately very rare to find and I can't direct you to one that I know.
On the other hand, the Daoist philosophy has a broader map of energy than the Indian yogic teachings: there are many Daoist teachers who deal with energy all the time - from traditional Chinese doctors and acupuncturists to Ki Gong and Tai Chi masters. You probably can find one of these close by. And of course, there are energy intuitive like Reiki healers who may be able to help you understand what is happening.
From a Western perspective, I would speculate that you are stimulating energy through either nervous system release or a fascial pressure release. Pressure on the tissues is known to send a variety of signals through the body: piezoelectrical signals, mechanotransduction signals and water state changes which may also have a cellular signalling effect. It is possible that some of these signals are reaching the muscles and activating them, although in a somewhat random manner. (This could be similar to the way most of us can sit, with one leg on tip toes, and cause our leg muscles to move the leg up and down quickly, without any volitional effort: the nerves just turn on and off the muscles repeatedly. Maybe you are doing that somehow.)
Hopefully, some other readers with more experience and depth in this area will share their thoughts.