Normal Pacman behavior in response to a "possible predator" that in their instincts means them harm. Some have it more pronounced that others; but it's the way it is. Recommend get some long (12 in. plus) plastic tipped stainless tongs.

Are you feeding frog to satiation every couple days? Maybe it's trying tot tell you it's hungry. Also, never approach any Pacman or large frog like an ABF from the front, it could result in a nasty bite .