He could have some problem or swelling which is making him uncomfortable.....or maybe he is too hot and is gaping a little bit?
Could you get close up pictures and give some details like temperature?
Honestly, Isis does that but I chalk it up to having to force meds into her when she was sick a couple months ago. I've taken her to the vet and he didn't seem to think it was a problem but its not something I like to see and I too would like to know why she does this!