I don't know. There doesn't seem to be any rhyme or reason to it. Sometimes it happens when I have been talking more than usual, or have been out in the air, or doing something strenuous. It's weird, isn't? There doesn't seem to be any one thing that causes it. Sometimes it is embarrassing if I have to speak. Last spring I had to present a scholarship at the local high school and it happened after I walked into the school from the parking lot. I had to park at a short distance from the school, so I don't know if that had anything to do with it. Fortunately, I was OK by the time I had to make my presentation, but it was a worry when it happened.