After the booing, the threats to send him to the minors, the failures at the plate, letting his team down, talk of the Yankees getting out of his contract, Giambi certainly would have motivation to succeed, very strong motivation.
If he knew that he could not succeed without some type of help from a drug I don't know if he would have the strength to resist the temptation.
He didn't resist before, and it is just possible that he decided to do what whatever was necessary to get back the way he was.