It's less that Bell's excited, but she is visibly frightened. The hare is so frightened that, after a close call of almost being sliced, the berserker strength activates. Bell pushes herself off from Shura's shoulder, doubles in size, then starts downright trying to rip open the throat of the orc that attacked her.

Strange reappears a few feet away from Shura a few seconds later, unwinding the scarf from his neck. There's a brief moment where he just stares at Bell before he remembers right, they're trying to kill me, and bends down to focus his magic through the earth.

"Well! That's new!"

This is pure talking to cover up the sheer terror at the situation they're in and the sheer confusion at the fact that his animas is now a murder bunny.