My money's on a "deadman timer" program that triggered after he failed to log on to the system for a while. Whatever else he did with the "changed settings," they would been utilized by his bomb as well.
According the
Fannie Mae Contractor Indicted For Logic Bomb he wrote it that afternoon from his laptop which was not confiscated.
------"The discovery occurred on Oct. 29. Makwana had been terminated as a Fannie Mae contractor on Oct. 24, around 1 or 1:30 p.m., the affidavit says, but his network access was not terminated until late that evening."------------
--------"Makwana was not required to turn in his badge or Fannie Mae-supplied laptop until the end of the day on Oct. 24. According to Nye's affidavit, it was during that afternoon that Makwana is alleged to have planted the malicious script." ---------
see #53.
This account says he was fired on or around Oct. 10-11!! You would think that they would have taken his commputer away before Oct. 24!!
(Unless, of course, he was being set up to be a patsy)