He ws holding Victor and his hand was on one end of the wormhole when it closed. Thus the wormhole shut with his hand on one side and the rest of his arm on the other.

Like in Star Gate season one episode two. They opened the wormhole and closed it with part of a character's head in it, and part of his head was missing after it closed. Needless to say he died, but Wiper would only lose his hand.