How much are you feeding him? When mine was a baby we would give him 15-20 1/8inch~ish -1/4th inch crickets at a time. Either that, or it could be he was drinking. Or it could be a cricket antenna got stuck on the side of his mouth, which does happen if the cricket is too big. Sometimes mine just sits with his mouth open if he is startled or surprised (he puffs up and opens his mouth wide.) It could be that he was startled by something, but as he is doing it all the time after eating, it sounds more like a food/drinking thing....