Obligatory answer first: plumbers. Second, perhaps some megafauna predators that the species outlived, especially on islands? But then, wouldn't the species have begun to lose the armor without selective pressure? Maybe the species began to see the armor as "sexy." I mean, it stands to reason that whatever adaptations increase survivability should as a side effect be seen as attractive features in a mate, right? Then again, without predator driven selection as a factor, wouldn't that have created a feedback loop like what supposedly happened with birds of paradise? Maybe their lifespans were long enough to prevent that by allowing multiple generations to compete for mates at the same time.
I probably should have just stuck with my first answer.