Leviticus clearly had yet to learn his lesson when it came to snow, ice, and the dangers they both created and concealed. He wandered North, toward the marsh that he and @Sawyer had skirted round when it's fringes proved too hazardous. Levi honestly didn't think about all that slippery crackly stuff extending this far East, especially when the trees never stopped like they usually did when the land stopped and water began.
So when he started to hear those weird, echoing sounds below his paws, Levi only half understood what was happening. Confused and unmoving, his head twisted back and forth as the ice beneath him cracked and buckled.