This is the first time that I’ve added a curved tail to a speech balloon. In Manga Studio EX5 this kind of tail is known as a spline. Mathematically, a spline curve is one that moves through a given series of points. Because of lack of headroom in this frame, a straight tail would have to emerge from the side of the balloon, which would look rather awkward.
No wonder Simpson looks so pleased with himself, he’s the first character to get a spline bubble in this comic strip. I don’t blame Waterton for storming off indignantly.