The Casa Chica fire started in an earth-plastered strawbale wall behind a cob fireplace which had held several sustained fires for a couple days... and between fires, the hot coals. The structure and the fireplace were about six years old, and the fireplace had seen this level of use many times previously.
Whether the fire in the wall started due to the straw being heated until it reached a flash point, or if combustible gases found their way through invisible fissures in the cob and ignited in the wall, or whatever, doesn't really matter. What matters is that the convention - which also applies to adobe walls - of leaving a vented airspace between the fire chamber and the wall wasn't followed.
Building a safe, well-functioning strawbale structure (or cob, or straw-clay, or wattle-and-daub, or rammed earth, or papercrete, or timber frame, or cordwood, or anything) means knowing more than just how to stack bales (or stomp cob, or make slip, or weave saplings, or pound dirt, or pulp junk mail, or notch a log, or stack firewood.) If you don't learn how to do all of the things entailed in and around your building, and understand why so many of them are generally done in particular ways, there can be significant consequences.
Several hours after the last fire in the fireplace (ie, the next morning), some smoke was noted coming out of the wall around a couple protruding viga-like logs. That was the first clue that anything was happening.
Starting at the end: This is the back of the cob fireplace (note the delineation of the lifts), after the fire, and after the unburned remains of that section of the strawbale wall were pulled out. There were areas of the cob, even here many inches from the firebox on the back, that seemed to have almost vitrified. The supposition is that probably happened (if it indeed did happen and wasn't some other phenomenon) sometime before the wall burned.
You can take a look at this structure in the cob section to get oriented, if you want. On the left side of that photo, on the wall, you'll notice a spiral; that's the part of the wall that burned. The fireplace is on the other side of that wall.
The photo below was taken from a vantage on the other side of the fireplace, looking back toward where the photo above was taken.
Note that the log - which is a big sucker - is charred more than halfway through. Not burned, but charred. This isn't the work of flames. This charred part of this log was embedded in the wall, directly against the straw. The earth plaster was applied over the straw, and stopped directly against the log.
You can see this log, to varying degrees, in the following photos of the rather boring firefight.
Opening up the wall (as Tony, who is now actually some kind of bona fide forest service firefighter dude, is doing here with a pickaxe) gave a lot more oxygen to the smoldering fire, which consequently produced an awful lot more smoke. Note that most of the straw coming out of the wall is nice and yellow, and not burning.
There were only a couple very brief bursts of pathetically weak, small flares in the wall as the straw was pulled loose, and they both self-extinguished almost immediately.
Loose straw is, as we all know, typically very flammable. However, in this case, as shown in following pictures, even the still-smoldering straw fully removed from the wall into big loose piles didn't do anything more than sit there and smolder. The wall was being continuously sprayed with a garden hose, and the significant wetness from that process may have been why there just weren't any impressive flames.
Pete digging around in there bare-handed. There's more steam in this picture than there is smoke. Spraying water into the smoldering bales produced copious amounts of steam.
It should be noted that the fire was contained to one section of the strawbale wall about two bales wide, a section of straw-clay wall, and that big wooden post - despite having potentially been smoldering for hours before anyone noticed. Whether or not it started ten minutes or ten hours before it was spotted, the fire didn't travel laterally in the bales.
It should also be noted that the firefighting effort did a lot more damage than the fire did... but if the fire had been left to its own devices, it would almost certainly have worked its way to the roof and ended up making a total loss of the structure.
Luciano having a turn with the pickaxe. Note the growing pile of bright yellow straw on the ground.