The 2nd post shows he had 16 cells per shelf 2 rows of 8 if they are 280Ah batteries that about 12lbs each I'm rounding up that would be 192 lbs on each shelf. I have put well over 600 lbs. on 3/4 inch particle board shelves are built like these without any issue of collapse. I know the ones that are sold at Home Depot, and use to be sold at Costco both had 1200 lbs ratings. I don't believe these ratings are bogus they would be liable they are testing these shelves. I have even had oil soaked into mine had a 350 small block Chevy engine sitting on it for 5 years.
I think something got hot his circuit protection didn't work started a small fire that caught the wood above it once that glue in the wood starts to fuel the fire it gets hot quick.
OP is in Australia, his suppliers are not the same as your suppliers. the shelf material may be thinner - we don't know.
From other posts, OP has a long conversation about humidity forming in the garage affecting equipment - that doesn't mean water or humidity affecting these shelves but sure is concerning.
The dates of the OP's posts about purchasing the 93 550W panels are in 2022. Not sure about the cells. From the pics and postings he used Daly BMS - or at least one is Daly. I have had some issues with Daly and their factory settings are not ideal. others have posted issues too.
His Inverter appears to be an MPP 11kW 230 volt - and additional SCC MPP PM60's which are only 150 volt max. some discussions on 2s 4p strings
Can you imagine how many strings? - only 8 PV at a time to wire up all 93 panels?! and if this is how it ended up in the actual system the amperage would be something! We are not seeing the whole picture but starting to wonder about high amperage PV strings, and loss of control over the charging of a pack.
This could easily lead to an over-voltage situation, cell venting and ignition. if a shelf failed before or after the fire started is not clear, but seems unlikely to "just happen" out of the blue.
51kW of PV at 51.2 volts is 1,000 Amps!
even with ten packs, 100 Amp each.
now what happens if one or two of those 3P breakers trip - we see some tripped - that 1,000 Amps is going to fewer packs, and much higher amperage.
Also being southern hemisphere, they are in late summer conditions right now, not winter like those of us in the northern hemisphere. The garage may be hot to begin with, and then high amperage charging could add to that heat. OP notes a window A/C unit in one post.
I highly doubt the shunt is the cause.