With Tesla quality control, a fire is just a bug, until it kills the truck.
Edit: and before an Elon bootlicker comments that it was fireworks in the truck bed -- I get it. The joke is based on Tesla's reputation for poor build quality, and yes, it has that reputation.
Fireworks are made of flammable metals or other flammable elements that burn differently depending on their makeup. Electric batteries contain some of these, so it stands to reason that you would see something like this if one exploded. Especially with thousands of individual cells.