The nCipher Advisory #13 cited in your securityfocus.com link
contains the explanation of the vulnerability (in the section
"Cryptographic details"). The CBC-MAC algorithm works similar to
the CBC encryption algorithm, but only outputting the final
block (or a part of this). Each block of the plain text is XOR-
ed with the previous ciphertext and then encrypted. This means
we XOR the first block with the initialization vector before it
first comes into contact with the block cipher. So changing only
the first block together with a corresponding change in the
initialization vector will mean that the same value is input to
the block cipher, which gives the same resulting authentication
tag at the end (which does not have to be changed). If the
initialization vector is fixed as zero (or some other constant),
there is no way for the attacker to change it together with the
first block, and this kind of attack is not possible. As an
added bonus, we have one block of data less to transfer.