M.R.
For a minute i thought you were talking about my husband...I think this is something they do when they are nervous or stressed. My husband has been doing the same thing since he was a kiddo and is 28 yrs old now. I have been trying to get him to stop since we started dating 6 1/2 yrs ago. I don't think there is anything for a mother or a wife to do about this, other than when you see him do it bring it to his attention - he might not even realize he's doing it.