Powell went to the UN Security Council just before the invasion and put his credibility on the line that Iraq's WMD program had to be stopped. He reportedly had long talks with Tenet beforehand; Tenet told him that the WMD intel was a "slam dunk".
After the invasion, when the WMDs were not found (overstatement, I know), Powell had egg on his face (he had portrayed himself as the restraining influence in contrast to the Cheney / Rummy crowd). That summer, post-invasion, is when Wilson started mouthing off; and if Wilson were to be believed (he turned out to be a huge liar), Powell had been fooled. So I do see a motive (back then) for Armitage to have sought to discredit Wilson.
For Armitage to have been such an amateur to have made the same leak three times and then forget to tell Fitzgerald the whole story is very hard for me to believe of a very smart guy.
I agree it's hard to imagine Armitage as a big dumb girly-man, gossiping to who will listen just to impress someone, especially a reporter.