Gas grill overheated
I might have an Idea to what happened. All grills have a drip pan, most all new grills have a drain for grease that falls from our savory meats. Well, if, (not saying that you haven't) but if, you have not cleaned the drip pan, there is still grease left over that gets absorbed by food droppings. IF this is the case, you had a ticking time bomb of a grease fire. The grill did not overheat, the grease caught fire. You said your grill was a year old; so the regulator did not go full blast and when they do go bad they do not let gas pass through.
Not trying to preach to no one because I have done it myself, never leave your gas grill on and unattended. I hate to say it but even the owners manual says it.
I hope I did not come off the wrong way and I hope this might give you an idea/thought of what happened. Read More At : Stainless Steel Gas Grill...