Why does my dog destroy her kennel?

My 2 year old German Shephard just started to rip at her kennel to the point where her mouth bleeds. She has been going into her kennel for the same amount of time for almost two years and just this week started to chew it up. She has never seemed unhappy about it and goes in with out trouble each time I ask her to. It just seems like all of a sudden she cant stand it.