Dude, I can't even imagine the anxiety you have right now. Just keep your head up, and remember anything could have happend to cause this, and no one knows if you are actually at fault or not. So don't just assume you are at fault. The facts are - they had the vehicle at your shop, and 6 days later the vehicle caught fire. I agree with Premier that it is important to express concern with how the situation looks, and offer to make it right just based on that alone.
Plus, one less leaky KIA on the road isn't necessarily a bad thing anyway.