Sorry - just saw this! So think of the sbox as a function. What happens if you apply the function over and over again? The order of a group like that would be

Sorry - just saw this! So think of the sbox as a function. What happens if you apply the function over and over again? The order of a group like that would be small (particularly since the sboxes are random). Also, the output space of the hash function would be quite restricted as a result of the group order being low.