http://www.slate.com/id/2240157

Another possible explanation would be that engineers possess technical skills and architectural know-how that makes them attractive recruits for terrorist organizations. But the recent study found that engineers are just as likely to hold leadership roles within these organizations as they are to be working hands-on with explosives. In any case, their technical expertise may not be that useful, since most of the methods employed in terrorist attacks are rudimentary. It's true that eight of the 25 hijackers on 9/11 were engineers, but it was their experience with box cutters and flight school, not fancy degrees, that counted in the end.