I figured out why my frog does this! It is only if I don't let go of the food fast enough after he has grabbed it. In this video I was using tongs/hemostats that you have to 'unclick' to release which takes a second or two, so I was still holding on to the worm and the frog sensed that so he warned me to let go. I'd hate to find out what would happen if I didn't let go He's ferocious!