I was chatting with a friend last night who said the same thing happened to him on the M25 a few years back. He was driving a Mercedes Sprinter and he noticed black smoke billowing from the back of the van. By the time he got over to the hard shoulder and got out of the van, the tyre burst into flames. Luckily for him, someone pulled over with a proper-sized extinguisher and was putting it out while he managed to remove the wheel; he said even while extinguishing, it kept reigniting, and when he managed to take it off and threw it aside, it burst into flames again. The problem is when you're driving, it's cooling the tyre, but when you stop, the heat builds up, and that's when it bursts into flames. I think the speed you're travelling at is an essential factor. He hadn't realised he had a flat tyre until he saw the smoke billowing.