One of my dogs has started chewing the corners on 2 cushion covers. He's never touched any before ( we've had him 4.5 years) and it's only 1 design that I bought 3 months ago to replace a couple of worn ones. He's just started doing it in the last coupleof weeks. I now need to replace the 2 but want to try and work out why to try and make sure the new ones don't get the corners chewed off before I do. Any ideas? His routine and diet haven't changed.