Same reason so many trucks catch fire! Extra heat generated via engine and insulation on body and electrical not up to the requirements. Saw a Irizar yesterday in Skinny Dog red that had an electrical fire, which may have also been the result of the one on the Sunnshine Coast during the week.
Brakes overheating as well and drivers, who also may not have a lot of experience after they have been recruited from a retail environment or unrelated work to "drive our bus"
Fast becoming the time when Fire detection and also suppression will become a standard build feature in buses. In Brisbane they have fitted up suppresion to a lot of the early gas buses and are moving into later models now, it is just the cost of fitment- but if it stops a fire which destroys a $400K bus and all its passengers, it is worth every cent!
Gas engines from my experience havn't really had any increase in fire "worthiness"
Mainly fires still appear to be contributed to a lack of engine cleanliness and maintenance generally.
The gas will come out at a controlled or slow rate, and if it did come out in a big rush, as in a rupture of a cylinder, it would just blow out the flame, as the gas needs to be around 15% of the air volume to catch alight.
The biggest danger is the bottle as it can turn into an unguided missile