I have only witness once someone being slain in the Spirit...never experienced it myself...but..
If God were to suddenly open us up and expose us to His infinite glory and majesty, none of us could handle it. It would completely overwhelm us and overpower us. Our systems might shut down at the enormity of it, and we wouldn't be able to stand up under it or bear it. God is so much bigger than any of us are capable of comprehending, and God is probably protecting us by not revealing the fullness of Himself to us while we are in our weak, mortal bodies. So when the awesome power of God touches a person in a special way, it can easily cause a physical reaction in the frail human body and the frail human nervous system. People's hands or bodies might shake, their knees might get weak or wobbly, they might fall to the ground, and so on. Those of us who have never experienced the Holy Spirit in such a tangible way might think that a person is faking this or is demonized or something, which can sometimes be the case. However, the power of God is real, and God sometimes "touches" people in special ways.