If I'm reading the timeline correctly, Zach was recruited during the mid-season hiatus--which was right before teh strike. However, he does show unexpected incompetencies during Max Keenan's trial, which could be signs of distraction, and it's obvious from the beginning of the season that being in Iraq really unsettled him. It could be seen as an asspull, but there's also the element, when it's revealed, of "Of course, he was so messed up by Iraq he was easy pickings." And he was always looking for approval from Booth and others. It was Sweets or Miss Lillian who said it was the classic weaker personality overwhelmed by a stronger personality.
That's part of the tragedy, everyone at the end suddenly seeing how they contributed to what happened. Hodgins couldn't have known that Zach was taking his rants at face value and accepting everything, Angela was focused on dealing with her husband, Booth was always weirded out by Zach, likewise Cam, and Brennan had her father's and brother's criminal problems to worry about. Gormagon probably scoped out the situation at the Jeffersonian and saw Zach as the perfect target.
It could have been set up in a neater fashion, but I see it as perfectly, tragically plausible.