From Flag Officers 4 America:

As the principal military advisors to the Commander In Chief (CINC)/President, the SECDEF and CJCS were the two top military officials in a position to recommend against the dangerous withdrawal in the strongest possible terms.  

If they did everything within their authority to stop the hasty withdrawal and the President did not accept their recommendations, then knowing the disastrous consequences looming, the retired flag officer signers believe these top military advisors should have resigned as a matter of conscience and public statement.