The ones trapped below decks likely died from smoke (hopefully).
If there was a propane leak, the gas might have leaked down into the sleeping quarters and succumbed the sleepers, or worse, ignited while they slept.
IF propane heaters and cooking equipment were in use and a leak developed, the propane gas would tend to sink (heavier than air) and might tend to accumulate in bilge spaces unnoticed by passengers and crew in a boat not equipped or with a malfunctioning detection system...
Once bilge spaces are charged with gas/air mix all that’s necessary for an explosive fire is the gas/air flammable range meeting an ignition source...