Sounds like a nervous habit, or you are doing it because you can.
Only been happening for the last couple of months. Among the people who have pointed it out is my mum and sister who said I don't usually do it. I also don't do it all the time just occasionally
People do move (or not move) their jaws differently when then speak.
Look in the mirror and look at your mouth and jaw as you speak. You may be able to correct this yourself. It may take some time to control our jaw, if you have been doing this for a long time, but it can be corrected.
It may be a bad habit that you have gotten into as a child that was never corrected.