I’m so sorry for your loss. But, it could have been much worse. Could have easily burnt your house down.
It sounds like it was the BMS to me. I have had several BMSs malfunction on me. Mine didn’t catch fire but, the PCB board looked melted and burnt a couple times. Twice on a Onewheel, one on a electric skateboard. Since then I have removed all my BMSs. I balance charge externally with a hobby charger, and monitor individual cell voltage during use.
Besides burning your house down, BMSs are just one more thing to fail. I had to walk home more then once due to a failed BMS. I don’t want to have to swim back to shore because of a BMS on an e-foil.
Thank you for sharing this experience. We all need to take this as a lesson learned. Everyone needs to think safety about how you charge and store your batteries. They need to be in a fireproof enclosure whenever they are unsupervised.
I recommend building these batteries with individual cell fusing. Fuse the battery wires, and fuse the balance wires. That’s how I built mine but I still take the fireproof box precautions.