Without any details, who knows?
Was the Bolt even the cause of the fire? Was its battery pack the cause? Was it the same cause as the other 5 identified Bolt battery fires? What was the battery's state of charge at the time? How long was it that SoC? Did it have the temp fix applied or was the max charge level limited to 90% or hill top reserve on? What model year was it? (All '17 and '18 were included in the recall but only a subset of '19 were.) Was it part of the recalled population?