My horse just developed awful manners on cross ties! help!?

I've had him since he was five (he's ten now) and has always been an angel on cross ties. Within the past week or two, however, he has been getting progressively worse when left alone on them. He mostly just paws at the ground and dances around a bit, but I left him alone for two minutes yesterday and he broke his halter and got loose. Then I put him on cross ties inside and he kicked and broke a muck bucket. This was all after I rode him, although he was pawing at the ground before I rode him as well. I'm pretty sure it's not the flies, I've made sure he has tons of fly spray on. He behaves fine as long as someone is doing something to him- he is perfect while being groomed, tacked up, and bathed, it only happens when he's left alone or I'm not paying attention to him. What could have caused this and what can I do to fix it?