I am brand new to the off grid and LiFePo battery environment, but I was in the trucking industry as a mechanic all my life (almost 40 years) and have seen a lot. I can almost guarantee and bet a steak dinner that Steve S. and others are exactly correct with their assessment. With the thin plastic cover around the cells, and direct contact with the metal frame there was a short. It would not take long at all for the road vibration to rub through the plastic cell cover, no matter how tight everything is. Metal flexes going down the road, especially true with something thin like Uni-strut. Build the metal battery box with something more substantial such as 1/8" plate and weld the joints. Insulate the bottom and sides with some thick plastic or rubber. Flat mud flaps make great material for this, (not the kind with astroturf to hold down rain spray). Use a non conductive battery hold-down clamp or at least put plastic under a metal hold down clamp.
Also use flexible cell connectors since the threads are so fragile on these cells.
If the batteries are installed inside the vehicle, I believe you would have more wiggle room with your design since the rubber body mount cushions would absorb some of the minor vibration, but bolting your battery box directly to the frame calls for extreme measures.
Wire insulation is also critical, especially when crossing the edge of a frame rail, going through metal body panels, etc. I always secured battery cables as much as possible against anything I could and double or triple insulated against edges with heater hose or similar. I have even seen plastic wire loom rub through the insulation on smaller wire harnesses, and then the wire would corrode when the salt spray in the winter has a chance to work on it.
I know this is a late post, but like I said, I am new here and this is my first post, but I wanted to add my 2 cents for what it is worth.
Also, I have learned a LOT from reading through various threads and obviously there are some very knowledgeable and smart folks here, and I hope to learn more! I am building my first set-up in the garage where vibration will not be an issue, but fire certainly can be.