Powerline scoffs, but the idea sounds reasonable to me. Based on their early morning demeanor and acquaintances' descriptions of their personalities, I wouldn't be at all surprised if they thought they were just doing a trial run, or carrying explosives for someone else.
UPDATE: More on the dupe theory here. HT: RealClearPolitics.
UPDATE 2: The Mirror weighs in, with an explanation of exactly how they may have been duped:
Our source disclosed: "The theory that they were not a suicide squad is gathering pace. They were the weakest link.
"We think it's possible they were told that when they pressed buttons to set off timers they'd have a short time to abandon the bombs and get away before the blast. Instead, the bombs exploded immediately."
That makes more sense than my early speculation about a trial run. HT: NYGirl via Alarming News.