The bar burned because of Alva, but Alva didn't do it. Consuela set him up - her own son. He got convicted: arson for the insurance money. But he was innocent. Of that, anyway.
The bar was supposed to be built on top of an ancient pueblo, but it wasn't. This was a secret hiding in plain sight. The north wall mural the WPA guys painted apparently told the story.
The photo was taken after that gas furnace was put there on the ceiling. The upper right corner of the mural shows where they're going with the bricks. You couldn't see that part unless you stood under that heater with a light.
The night of his father's funeral reception in the bar, Alva took a sudden intense interest in the mural. He had planned on coming back. He was caught running from the fire.
I'm not sure why it was so secret: that a pueblo wasn't where it was supposed to be - or that a bar wasn't. But it was a big enough secret that Consuela burned her own bar down to keep it.