I just got it down and clearly what happened was what Larry surmised.  I could only see maybe one open hole on the center burner.  I am sure the gas was backing up and coming out around the orifice or more likely the venturi opening.  Eventually it probably collected enough under the control panel to find a source of ignition from one of the other burners inside the cook box and "Whoosh".  Funny thing is even with plastic firmly melted onto the valve stem, the valve operates smoothly.  
This is clearly one of those grills where the guy does zero maintenance.  No cleaning and no parts replacements.  Just use it until it quits.  I am sure that middle burner was completely out for a while and the front and back ones are probably half clogged as well.  It couldn't have been functioning very well for the last year or so of it's life.
Now for the really strange part.  I would have called this grill a Gold C.  The lid is stainless and clearly shows GOLD on the Weber logo.  I was under the impression that the extended frame made it a "C" version.  But, either I am completely wrong on both counts or something is amiss.
Here is the grill link.  Look at it and tell me what the model designation should be.  
https://www.facebook.com/marketplace/item/267317117509929/?ref=messenger_banner
Now, bounce that off the serial number sticker and tell me what you think.