While enjoying one of my favourite snacks the other day, (Hula Hoops) I suddenly had this thought; if ever a metal nut is hard to loosen with a wrench, usually a a couple of sharp hits with a hammer is enough to break the grip of whatever is holding it - usually rust. Similarly, crunching on foods which can impart small shockwaves to a capped tooth could have the same effect and cause the hardened dental cement to fracture and loosen the crown. (cap)
I Googled and found a site which warns against eating certain foods if you have capped teeth.
Just a thought.