Computer chair goes bonkers

Last night, around 10:30, my wife was using the computer. The chair for the computer is a swivel type, average backed chair with padded arms that are firmly fastened but do have some flexible movement either vertical or horizontally. I had just stepped out of the den on my way to the kitchen when my wife, directly in front of me, in the computer chair, which is in the library, exclaimed, "What the heck?" The right arm of the chair was moving in an up and down motion. Though my wife was sitting in the chair, she was not making contact with the right arm of the chair. After moving up and down on its own for three times it settled into being a plain arm on a chair again. Not a big deal but still a bit of a wonder. I've tried sitting in that chair and moving and wiggling all about, but I have had no luck making that arm on the chair move on its own. I have to physically move the arm up and down. So how did it manage to do it on its own, with witnesses no less. Interesting, don't you think?