Computer chair moves on own

Its been a pretty quiet month. We have had considerable rain and  the weather has waxed between hot and very cool so much that a person doesn't have time to get accustomed to this weather. Today I encounted a rather strange occurance that could be totally mundane or maybe not. I was at home most of this morning; my wife was running her store in a neighboring town. Both of my schnauzers were taking a nap in the dining room on their pillows near the heat registers (no heat in them today, too warm outside). I had just come through the diningroom from the kitchen and made the righthand turn into the library where the computer is located. I had planned to see what, if any, E mails had accumulated. So as I rounded the doorway into the library the computer chair moved as if someone had just gotten up out of it and the right armrest made a definite tilt to the right, The armrest on the right side of the chair is not on very tight and it moves or wobbles when you use it for support in leaving. The movements were not great or exaggerated but the change in attitude of the swivel desk chair was evident. It got my attention  but it need not necessarily been paranormal. Though I have no concrete idea why the chair moved in that fashion just as I entered the room. But as I have said before, if something unusual occurs here I will report it.