So if there were passengers on board -- say while the APU is starting after landing -- it sounds like this fire was sufficiently large to prompt an emergency evacuation (e.g., pop the slides), right?
The fire appears to have been fairly small, damage-wise, but the smoke and fumes generated that appear to have quickly entered the passenger cabin would be what would've prompted what you describe. But that's my own nitpicking, really, as any fire onboard of practically any kind would likely prompt the above, especially if it's an unknown of size/intensity/location, and can't be immediately extinguished.